I don't think Castro moves - would be too tough to make that transition in season without him ever playing there before. His status depends on what they think of Russell - they are going to try him at 3rd in AAA to see what it looks like - he is the better defensive SS, though, and Castro will be dealt before Russell if they think Addison can hit in the bigs. I don't know if it will be this year, though, unless they get a sweet deal - which they haven't gotten yet. There were a bunch of rumors that they were looking to get young pitching from the Mets for Castro in the offseason, but nothing came of it, and what I read was that Epstein wasn't getting the value he expected for Castro, even though he is still really young and on a great contract.
Baez is still the 2B of the future - in the meantime, TLS is young and serviceable, same with Mendy, until they figure out if Baez can fix his Ks.