Christian McArdle wrote:
I cannot change the old backbox.


There's no can't about it. It is not acceptable to have such a large gap.
You must put new backboxes in, closer to the new surface. It isn't that
difficult to do. The gap is so large that the old ones can stay buried
beneath.

I'm about to leave a small gap, as changing from a painted wall to
a tiled one. So what is an acceptable gap? Less than finger width?

Ben