I recommend you, to sell first, and then buy. You and anyone else can test this by their side btw, and really, as a manager who kept payers till retirement, thats what I recommend you.
Okay. I'm gonna test it for you. First thing i sold 5 sub. players ( 3 players were attackers as ST) from my squad and then i've signed with new player who is a FT. We'll see how he will fill the team slots.