In a 1960's home I replaced a circuit that had old, ungrounded wire
with 12/2 NM cable, and phished it up through the wood floors into the
existing handyboxes. All went well so far.

Unfortunately the old handy boxes aren't big enough to hold both the
incoming 12/2, and outgoing 12/2, not to mention the grounds that are
going to be connected.

So now I need to remove the older, smaller handyboxes and while I
managed the wiring part without damaging anything, I don't know how to
remove the existing handy-boxes to be able to install remodel boxes
without creating a mess. Is there a method that will allow the least
breakage of the drywall so that I can patch it up easily?

Thanks.

--
Himanshu