Replacing pattress boxes.

[GagsPritch]

Recently my wife asked me to replace all the mounting boxes in our house. They're all pretty old and it seemed like a good idea. I bought new boxes for all the sockets and light switches in the house and although I'm not an electrician I did go to college to do level 2 electrical installations so thought this would be easy. It turns out that all the premade holes in the new boxes are in  different positions to the old boxes. So it's not just a case of removing the old ones and screwing in the new ones. I'd have to drill new holes for every single one. The thing is when I started drilling all the plaster started breaking away in big pieces. I guess my question is can I just fill in the cavity with filler and screw straight into that? What would an electrician do in this situation?

Thanks in advance