Short answer: There are two: someone who joined a Medicare Advantage plan (or PACE) when first eligible for Part A at 65 can switch to Original Medicare within the first year and buy any Medigap policy sold in the state; and someone who dropped a Medigap policy to join Advantage for the first time can, within a year, get that same policy back if the insurer still sells it.
In the second case, if the old policy is no longer sold, the person can buy Plan A, B, D or G (or C or F if eligible for Medicare before January 1, 2020). In both cases, apply as early as 60 days before the Advantage coverage ends and no later than 63 days after it ends.
The trial right is one-time: it applies to the first Advantage enrollment, and the person must leave within the first 12 months. Medicare.gov notes the rights may last an extra 12 months in some circumstances; confirm with the state insurance department.
Practical point: calendar the 12-month date at enrollment. After it passes, a person in Texas who wants to return to Medigap generally faces medical underwriting.