Yes.  The Centers for Medicare and Medicaid Services (CMS) provides a Special Enrollment Period (SEP) for people who left their Medicare Supplement (Medigap plan) -- joined a Medicare Advantage plan for the first time -- and within the first 12 months these people wish to leave the new Medicare Advantage plan and return to their original Medicare Supplement.

Medicare beneficiaries will use the SEP to leave their Medicare Advantage plan and return to their Medicare Supplement (or an equivalent Medigap plan if the original Med Supp is no longer available).

Important:  A similar "trial right" Special Enrollment Period is also available for people who enrolled into a Medicare Advantage when they were first eligible for Medicare and during the first 12 months, they wish to leave the Medicare Advantage plan, return to Original Medicare and join a Medicare Supplement.

Where can we find this Special Enrollment Period?

The Special Enrollment Period is defined in Chapter 2 of the Medicare Managed Care Manual "Medicare Advantage Enrollment and Disenrollment":

"For Medicare beneficiaries who dropped a Medigap policy when they enrolled for the first time in an MA plan, §1882(s)(3)(B)(v) of the Act provides a guaranteed right to purchase another Medigap policy if they disenroll from the MA plan while they are still in a 'trial period.'  In most cases, a trial period lasts for 12 months after a person enrolls in an MA plan for the first time. Such individuals would not be eligible for the special election period [SEP] provided for in the last sentence of §1851(e)* of the Act, because they did not enroll in an MA plan immediately upon becoming Medicare eligible, but instead had been in the Original Medicare Plan for some period of time. The right to 'guaranteed issue' of a Medigap policy under §1882(s)(3)(B)(v) of the Act would be meaningless if individuals covered by this provision could not disenroll from the MA plan while they were still in a trial period."

"Accordingly, there is an SEP for individuals who are eligible for “guaranteed issue” of a Medigap policy under §1882(s)(3)(B)(v) of the Act upon disenrollment from the MA plan in which they are enrolled. This SEP allows a qualified individual to make a one-time election to disenroll from their first MA plan to join the Original Medicare Plan at any time of the year.  The SEP begins upon enrollment in the MA plan and ends after 12 months of enrollment or when the beneficiary disenrolls, whichever is earlier. The effective date would be dependent upon the situation."

(Medicare Managed Care Manual, Chapter 2 - Medicare Advantage Enrollment and Disenrollment, Section 30.4.4- SEPs for Exceptional Conditions, 6. SEP for Individuals Who Dropped a Medigap Policy When They Enrolled For the First Time in an MA Plan, and Who Are Still in a “Trial Period”, 42 CFR 422.62(b)(4) (Rev. 1, Issued: July 31, 2018; Effective/Implementation: 01-01-2019) (Updated: August 19, 2011 (Revised: November 16, 2011, August 7, 2012, August 30, 2013, August 14, 2014, July 6, 2015, September 1, 2015, September 14, 2015, December 30, 2015, May 27, 2016, August 25, 2016, June 15, 2017 & July 31, 2018)))

*The last sentence in Section 1851(e)(4) of the Social Security Act, "Special Election Periods" notes:

"Effective as of January 1, 2006, an individual who, upon first becoming eligible for benefits under part A at age 65, enrolls in a Medicare+Choice [Medicare Advantage] plan under this part, the individual may discontinue the election of such plan, and elect coverage under the original fee-for-service plan, at any time during the 12–month period beginning on the effective date of such enrollment."

(https://www.ssa.gov/OP_Home/ssact/title18/1851.htm)

More about the two "trial rights" allowing a person to leave a Medicare Advantage plan and join (or return to) a Medicare Supplement

CMS provides two Medicare Advantage plan "trial periods" allowing a Medicare beneficiary to return to a Medicare Supplement policy with guaranteed issue rights (no medical underwriting) - and, depending on where you live, your state may provide additional guaranteed issue rights for joining a Medicare Supplement (for example, California, Connecticut, Massachusetts, Maine, and New York may provide Medicare beneficiaries additional rights).

Trial Right #1 - You join a Medicare Advantage plan when you are first eligible for Medicare and within the first 12 months, you wish to leave the Medicare Advantage plan, return to Original Medicare and join a Medicare Supplement.

If you joined a Medicare Advantage plan (MA or MAPD) when you were first eligible for Medicare (you turned 65) and within 12 months of joining the Medicare Advantage plan (your trial period), you decide to leave the Medicare Advantage plan, you are provided a guaranteed issue right to join any Medicare Supplement that is available in your state.

As noted by the Centers for Medicare and Medicaid Services (CMS):

"[If y]ou joined a Medicare Advantage Plan or Program of All‑inclusive Care for the Elderly (PACE) when you were first eligible for Medicare Part A at 65, and within the first year of joining, you decide you want to switch to Original Medicare - You have the right to buy [a]ny Medigap policy that's sold in your state by any insurance company.*

You can/must apply for a Medigap policy [a]s early as 60 calendar days before the date your coverage will end, but no later than 63 calendar days after your coverage ends.

Note: Your rights may last for an extra 12 months under certain circumstances."

If you are outside of this 12 month "trial period", you may be subject to medical underwriting (again, some states provide for more generous "guaranteed issue" rights).

Trial Right #2 - You left your Medicare Supplement and joined a Medicare Advantage plan and within 12 months decide to leave your Medicare Advantage plan and return to your Medigap plan.

If you were enrolled in a Medicare Supplement and leave your Medicare Supplement to join a Medicare Advantage plan, you will be granted a Special Enrollment Period to return to your Medigap plan (or other plan if no longer available) within the first 12 months of Medicare Advantage plan enrollment.

CMS allows the "trial period" if:

"[y]ou dropped a Medigap [or Medicare Supplement] policy to join a Medicare Advantage Plan (or to switch to a Medicare SELECT policy) for the first time, you’ve been in the plan less than a year, and you want to switch back - you have the right to buy [t]he Medigap policy you had before you joined the Medicare Advantage Plan or Medicare SELECT policy, if the same insurance company you had before still sells it."

If your former Medigap policy isn’t available, you can buy Medigap Plan A, B, C*, D*, F*, G*, K, or L that’s sold in your state by any insurance company.

If your former Medicare Supplement is no longer in existence, you will be permitted to join Medicare Supplement Plan A, B, C, F, K, or L "that’s sold in your state by any insurance company".  (Remember that Medigap Plan C and Plan F will no longer be accepting enrollments starting in 2020.)

You can/must apply for a Medigap policy [a]s early as 60 calendar days before the date your coverage will end, but no later than 63 calendar days after your coverage ends.

Note: Your rights may last for an extra 12 months under certain circumstances."

*Note: Plans C and F are no longer available to people new to Medicare on or after January 1, 2020. However, if you were eligible for Medicare before January 1, 2020 but not yet enrolled, you may be able to buy Plan C or Plan F. People new to Medicare on or after January 1, 2020 have the right to buy Plans D and G instead of Plans C and F.

(Source: Choosing a Medigap Policy: A Guide to Health Insurance for People with Medicare, Centers for Medicare and Medicaid Services,  https://www.medicare.gov/Pubs/pdf/02110-medicare-medigap-guide.pdf;
https://www.medicare.gov/media/9486)