I prefer this change, and in fact I've used something similar when a player generated a non-Thief Mechanician and wanted to be able to open locks. The Mechanician's Guild, among others, should be teaching Locksmithing.
It's always bothered me that Knock is so powerful compared with Thief. It makes a bit of a mockery of the locksmithing skill set, that it's uniformly inferior to just one of the tools in the Swiss Army Knife that is wizardry. I'd be happier with something like:
- A Lock spell applied to a lockless door imakes a standard difficulty problem of 3 dice for a thief, or maybe 2 dice for a thief and 3 for a non-thief.
- A Lock spell applied to a door already locked increases the lockpick difficulty by 1 die.
- A Knock spell makes something easier by as much as the Lock makes it harder, for as long as the Knock lasts (say, a minute).
- If the action of a Knock spell makes the lockpick difficulty go below the minimum (whatever that is) the door springs open.
- There could by stronger Lock and Knock spells at higher IQ.
- Probably just one spell per lock.
Maybe not exactly that, but something like it.