My baseboard radiator fins were installed so that now it obstructs air flow. They seem to still heat the room OK. The plumber has acknowledged that he did it wrong and wants to come to fix it. It will require draining the lines and then cutting the pipe and turning it a quarter turn. And re-soldering. But my husband thinks that re-soldering a wet pipe may compromise the joint and it could cause a leak to flood our new flooring. Is this a legitimate concern?

After draining, the heat of the soldering process will boil out any water.

You can't do it when its full of water, but as was said... the heat will evaporate the small amount that's there after its drained enough... I'd done it numerous times... and never had a problem... and I'm sure ma0614 has done it more in the last month than I have in my life.

Smoothy-I actually hate soldering, glad they invented Shark bites and compression fittings!!

I don't fancy it either... but except on the large diameter copper shower and tub drains with the brass fittings on older houses like mine (forget doing that with propane)... its generally very easy to do...

The plumber has acknowledged that he did it wrong and wants to come to fix it.
Be thankful for that and let the plumber do his job. Do not be concerned. His solder joints won't be any different than the first time he did it.