Its not a Volendam procedure, it is what the local authorities require. You will not have to "check in" or "check out". Are you in the same cabin both cruises? If you are, nothing will be required in relation to the status of your room or being on board. If not, you will work with your cabin stewards to transfer as quickly and easily as possible, and you will be issued new room keys, for your new room.
As to Immigration procedures, the most you will have to do is a joint procedure with anyone else in the same situation, (like you would at US ports) which may involve meeting Immigration officials somewhere, either on the ship, or ashore. However, as you have stopped at other Canadian ports, chances are good you will do nothing at all.
The best case scenario is you have to do nothing, you simply treat it as another port stop. This is common in most non-US ports.
In any case, you will receive detailed written instructions a couple days before you reach Montreal. You will have plenty of time to explore Montreal.