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

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.