How to remove metal back box that's cemented in?

[SimonH2]

Getting the box out is the easy bit - getting a new one in will be "interesting".
Unless the cables will pull back, then you have to fit the box over the cables - which means it has to be angled. That means the hole in the brickwork needs to be at least as wide/high (depending on cable entry point) as the diagonal of the box. For a 35mm box, that will typically be bigger than the size of the accessory being fitted. If cables come from more than one direction, then you're stuffed
And the holes in the box may not be in the right place for the cables which may be cemented in.

 

[DetlefSchmitz]

If cables come from more than one direction, then you're stuffed

It's surprising what you can do with a cut-off wheel on a Dremel to allow a box to fit from the front.

 

[sparkwright]

Before chiselling, you could drill some 5.5mm (approx) holes in the mortar around the box.

About 10mm apart.

Then chisel.

This will make the chiselling easier, quicker, and less likely to crack the surrounding brickwork.

Chain drilling.