A working key for each lock makes rekeying straightforward. Without one, the lock can still be opened and rekeyed, but it takes longer.
It also helps to know how many locks you want on the new key, whether any should stay on a separate key, and how many copies you need. If a smart lock is involved, have the app login or admin code available so the codes can be cleared at the same time.
For rented property, the person arranging the work needs to be entitled to do it. A tenant rekeying a unit, or a landlord rekeying an occupied one, should check the tenancy rules first. Ontario's rules are summarized on tenants and lock changes in Ontario.