You could use hardlinks for this, though (as long as both places are in the same filesystem). Then when you're finishes you just delete the old folder.
Huh. As a hardlink avoider, I never even thought of this. (I don't have a good reason for avoiding hardlinks - it's mainly just because I didn't grok them enough to predict their behavior. When you teach yourself, you gotta expect some gaps!) Thanks for the suggestion.
Whereas ‘cp -Rus’ achieves what lndir can, ‘cp -Rul’ does it with hard links.