The experience of glider owners of becoming EASA aircraft was that:
1) we had to G-register them. Not an issue for other stuff.
2) we had to bring all the paperwork together and show either in the documentation or by inspection that all ADs and so on were up-to-date.
3) this involved lots of form-filling... It had to be signed off by an approved inspector - in our case BGA inspectors with the required ticket.
4) we also had to pay some money, can't remember how much now, but the BGA were administering it on behalf of the CAA so your CAA charges might well be different.