From October 2023, new laws will be introduced that mean any career average benefits built up in FPS 2015 during the period 1 April 2015 to 31 March 2022 (the remedy period) will be converted to final salary benefits for the scheme you used to be in. These converted benefits will be called "legacy benefits".
The benefits you have built up in the FPS 2015 during this time will be called "reformed benefits" and will be held as an underpin on your pension record. At retirement you will be given the choice of legacy scheme benefits or reformed benefits for the remedy period. This is known as Deferred Choice Underpin (DCU).