Are you sure the valves are soldered on?
If they are, make sure and drain the water from a lower valve to ensure the water is drained from the upright supply lines.
Are you reusing the cabinet? If not, how about cutting enough to remove.

Yes they are ball valves soldered on. We will be selling the old kitchen cabinets so I don't want to do a lot of damage to them.

I much prefer to cut the pipe off and put couplers in than trying to re-solder the same joint.

Re-soldering can be touch unless you remove ALL the old solder. A very thin layer can remain, but it mst be VERY thin.

The clearance in a copper sweat joint is very small and critical to getting a good joint.

If lead solder was used in the old joints that is one more reason to put a coupler in instead of removing the surface coating on the old lead solder joints by re-heating them.

Contact your nearest local plumber, take his advise and he gives more better possible solutions

Here is a link that might be useful: Local plumber

thank you Brickeyee and Hadiya. I won't try to reattach the existing ball valves. A local plumber will be putting in the new gas lines and the new waste and supply plumbing but it doesn't seem worth while to contact him with such a question when I have so many experts here.

Is there any good technique for removing old solder from a fixture? These valves still look good.

Maybe I should ask how best to remove the solder that has run down the pipe from the valve, since I will want to solder on a cap until the plumber comes to work on the kitchen. I assume the remaining solder lead to a problem.

"Is there any good technique for removing old solder from a fixture?"

Heat it up and use a wire brush to remove the solder.

The layer left behind should be so thin that you can see the color of the solder but not feel any thickness at the edges of the solder spot on the surface.

I may be stoopid, but I use steel wool and/or fine grit sandpaper to hone down the surface and remove the excess solder.

It takes a lot of sanding to get all the solder off, and you run the risk of cutting the fitting out of round.

You do not need to have bare metal, just a layer thin enough to not interfere with a new joint.

For a while you could even purchase parts that had a tin coating on them in the solder cup. It had a self life before it corroded enough to require cleaning to make a good joint, but it was much easier to clean up than a plain copper surface.

The cost was not really worth it though, and I have not seen any in a few years.

HD was out of 5/8" caps so they sold me a couple of sharkbite caps to close off the supply pipes after I remove the valves, and the tool to remove them. Assuming I get most of the solder off, are these dependable devices? It only needs to hold for 6 weeks until the plumber rehooks up the new faucet. Yes, much more expensive but tough to solder down there and I can use again for future projects.

But if these are prone to suddenly letting go, maybe not. Anyone have experience with the capping devices?

'Shark bite' caps should be acceptable for a temporary installation.