Nice question!

What if you already have a master key that is used by a certificate aimed for another database (dbA) on the instance you're moving the dbB?

Just for clarification.

I think you should drop the dbA certificate (backup before) using the old master service key, then drop the master key and recreate with another password (same as for dbB certificate), and then create the new certificate from the cert and key files you moved on the new instance, using the new master key?

Regards,

IgorMi