Hey Riccardo,
I would guess, that it all depends on agreement that you signed with your ATO. Check terms and conditions, ask for refund, and have them sign your logbook. Nobody can take away your hours, but you may not be able to start in Italy in exactly the same spot that you finished in Greece, because there may be certain differences in training program.
Afaik you can't "transfer" exam passes, so if you started taking them with one CAA then you start from zero with another.
Have a nice day.