I've been seeing this happen on DSM to CLT flights the past two weeks as well, but it's not consistent. For my Mesa operated CRJ-900 on Friday the upgrade processed as normal as a CP, but my Mesa operated CRJ-900 for Sunday shows the "upgrades not offered" text with eight seats open. I'll try giving the CP line a call for my Sunday flight since I don't want to worry about OLCI on a Saturday afternoon.