There is no chance that, after selecting bcrypt, you will be forced to scramble to replace it with salted SHA-1 hashes. bcrypt is strictly better than what you're doing now.
You could do something similar.
PBKDF2(password, iterations=10) == PBKDF2(PBKDF2(password, iterations=5), iterations=5)
Thus you could, say, increase the number of iterations every month. All that said, you should still use bcrypt; this is just an interesting property IMO.