The easiest solution just seems to be allow them to change 1 spell when they complete an LR. So they have access to their full class spell list but (unlike a Cleric, Druid, or Paladin) can't retool their entire spell list overnight.

That feels a little generous to me though, but the headache of tracking 1 spell per week doesn't seem worth it.

Any thoughts on whether 1 spell per LR would be a problem? Or suggested alternatives?