You might want to check with support before doing it, but I think if you delete it from one domain, it's freed up for another one.
I'd guess that when you went to add the IP to the other domain, choosing your old one would be an option instead of paying.
Like I said, though... unless someone replies that has done it, I'd contact support first.
Save up to $96 at Dreamhost with ALMOST97 promo code (I get $1).
Or save $97 with THEFULL97.