Change from a mortise lock without buying new doors

You can have a locksmith rekey the locks, or you could even do it yourself with a rekey kit. To do this the cylinder will be removed, and the pins within it either replaced with different ones, or in theory they could just be swapped around. After this, the old key will no longer open the lock. You will need a new key. This must either come as part of the rekey kit, or one could be cut based on the new pin arrangement in the cylinder.

Do it yourself rekey kits are available for some lock brands online.

Schlage has a rekeying manual online that you can read and decide if it is a task you want to tackle or to leave to a locksmith. I would expect a locksmith to do it fairly inexpensively if you're able to bring the lock sets into the shop.

With that last bit of info, there is no way to key the Norwalk alike with the Schlage using the existing tumbler, it must be replaced with a Schlage or even Baldwin tumbler, which has a Schlage keyway. Yes you can still do it yourself but you must remove the tumbler, take it to the locksmith and have it matched up. This will only save you a house call charge. The screws to remove the lock are hidden behind the latch plate most of the time. Sometimes there are visible holes through the latch plate to access the set screws for the lock. Just the Norwalk tumbler needs to be swapped out, not all three. But the locksmith will need to know the key that it will be change too as well otherwise he may give you a 7 pin Schlage tumbler.