Yes, you'd have two for a moment.
So, in the Free Agent list screen, go to the Far Right, "Actions", (as Commish) Import Historic Player. His code is valenfe01 and use import year 1989. When he comes in, put him on the Dodgers, delete (or suspend, see below) the original one, go to the 1989 one and align the birthdate by a year. Thus, properly aged, but technically 1989 version of FV, playing in 1988. Sure, he will have two 1988 Stat lines (his real one and the one he earns for you in this What-If), but life isn't perfect...
On the recalc, I think the imported '89 one would become the '90 one after the World Series. So, if that's a concern, instead of deleting the original '88 one, just suspend him a year, and he can replace his doppelganger at year's end.
Alternatively, yes, you could copy every line in the Player Editor page to the 1988 one and keep the adjusted '88 one and then delete the '89 one you stole the info from off the Editor page. That seems like more work though
