Sorta, but chances are you'll be generating way less then an attacker would need to do. More importantly, you owe it to your users to keep their passwords secure.
It's unlikely that you have more users than the enumeration of the 128-bit key space (if MD5 was used). The slowness of bcrypt prevents the brute force attack through the key space for EACH user. That's N x 2^128 for N users, whereas to upgrade, you only have to do it N times.
