Anagram wrote in
:

Is it common to solder copper pipes under a kitchen sink, or is it
almost universal to use compression connections there? If you found
some already soldered there, and wanted to put new connections in,
would you use a torch to remove the old ones, then use compression
connections for the new ones? Or what?

Especially if pipes come out of a concrete slab foundation, and only
stick up 2 or 3 inches, so there is not much excess pipe length to cut
and start over.

Cut back to clean pipe. Put a sweat union on slab end and 4, 6, 12", etc.
extension in the other end of the union. Now you have all the pipe you want
to sweat or use compressions on with plenty of screwup room. If pipe gets
to short in future, just heat union and replace extension. Slab pipe never
gets shorter again.