Renee Jones has retired from acting. This was why she asked (and received) a storyline that ended Lexie's life. She did not want, down the road, to be contacted to return, etc. Just like James Scott (former E.J.)
Days is filled with recasts. Over and over. It is rare to have original actors for as long as we have had some (John, Abe, Marlena, Julie, Doug, Victor, Maggie, Steve, Kayla, Jack).
Currently we have Brady (formerly played by Kyle Lowder, who is now playing Rex, who was originally played by someone else), Abby, Jennifer, EJ, Chanel, Ciara, etc.
I would not worry, it is just one episode, & it is wonderful that Abe is getting this tribute. Lexie was a huge part of his life, it is fitting.