To re-register an existing microlight as group A is, in most cases, very problematic, as the safety standards are different. The only faintly easy switchover I can think of is the EV97 Eurostar.
Your best bet is to sell it, and to buy a cheap group A aeroplane with an LAA Permit to Fly. Some of those are down in the microlight running cost bracket - Evans VP1, Jodel D9, Group A Cyclone AX3, Group A Rans models and still good fun aeroplanes.
G