Balls, forgot the trick.

- Bend the backbox sides in a little to create a gap.
- Slide a pair of needle nose pliers over the backbox edge.
- Rotate/bend the pliers to pull the sides in.

Rotation is best because it is more controlled, no chance of ripping
the plaster out.

That is how I replaced 16mm & wood backboxes with 25-35mm for a
colleague.
- Box removed
- Screwdriver used to make wiggle space for cables
- Diamond cutter to deepen the box
- Cables slid through new grommet'd backbox holes, done.

Oh yes, I pushed the new backboxes into wet plaster so the next time
will be just as difficult :-)

Bosch multiconstruction bits are wonderful re easy drill into anything.