DC only seems to have room for three main Flashes at a time: Past, present and future. Sometimes that shakes out as Jay, Barry and Wally, sometimes as Jay, Wally and Bart, and most recently as Jay, Barry and Bart. They’ve got four major Flash characters to work from, and they want Barry in the “present” role…and now that they have a chance to rewrite history, they can get rid of the “extra” character.
That suggests two likely possibilities:
1. They erase Barry Allen’s grandson and reset his nephew to being Kid Flash.
2. They erase Barry Allen’s nephew and keep his grandson as Kid Flash.