Bastareaud wanted to leave SF, which is what made everything easier.
I don't really believe Contepomi's move, though. He is paid 300.000 euros per year at RCT and has one year left of his contract. SF surely can't match this salary (Bastareaud earns less than half of that), given the financial problems they have had of late, and Contepomi at this age probably cares (and perhaps should care) about the money more than he did four or five years ago.
Plisson has been a decent cover at 10 for SF, but having someone experienced to help him out for a year or two doesn't seem that stupid. The he can take over.
My club's training takes place right after SF's "Espoirs" squad (U-23 I think) an they don't really seem to be producing anything suitable of joining their first team atm (except for a Bastareaud-esque centre, but he doen's really know a lot about rugby yet).