I use a simple rule that seems to work somewhat in regards to renumbering.
I control the Israeli /16s blocks - i.e. 192.114-118.0.0/16. An
organization that uses a /22 or more (and use means more than 50% of the
IP address space is being used), is allowed to move around with it from
ISP to ISP (i.e. portable). If you have 500 machines - it is painful to
renumber (not impossible). If you have 30 machines and a /24, then
renumber - and I have reclaimed dozens of /24s in this fashion.
Hank