The pipes in the wall run vertical and are half-inch copper.
Attached to each pipe is a copper T. The horizontal leg of the T
attaches to a short piece of pipe going to the valve.

I want to disconnect the valve at the T and replace it with
another valve configuration.

Will the T be clean enough to reuse once I remove the existing
horizontal pipe?

Will I have to clean the inside of the T-leg? With what and how
clean does it have to be?

I assume that everything after that is as normal: flux, fit
together, heat, solder, cool, test. I know I can solder brand-new
pieces together, I did part of the orginal valve job years ago
and played around with some test pieces yesterday. I just don't
have a any experience working with used parts.

Related question: What can I use from around the house to
shield the surrounding wood and wallboard from the heat of the
torch?