I just had this happen to me; I have an award flight for August 2018 that includes the final legs of NAN - SFO - SEA on Fiji Airways. FJ switched their NAN - SFO flight from being on a Sunday to Saturday and auto booked us into the Saturday flight, but I had to call in to get the SFO - SEA legged patched up.
No idea when the schedule changed originally but glad I looked when I did so I could get my second flight updated to the correct day as well while there was still availability. I received no notification from Alaska, and just noticed it because I happened to be looking at the reservation. Wish there was some sort of email that came through to me though or alert banner on my Alaska account for things like this!