Actually, since new boxes are so cheap, you likely cost yourself more
time and money by trying to re-use the old ones, plus the cost of
welding wire, electricity, etc.

Then too, I like the professional appearance that new boxes contribute
to a wiring job, and I like to walk away from any job proud of what
I've done. I'm even particular about the cover plates that I use on the
boxes.

Then, that's just me.

Harry C.