Per the press release today, SkyWest has signed an agreement to operate the CRJ-550 for United Express effective December 2024. The initial agreement is for eleven frames with an option for more.
United Q3 Release
Per the fleet site and this forum, it seems like at least two are in OO’s possession. There seems to be some that have never finished the conversion to 550, along with 19 that never returned to service after the pandemic. I am guessing the initial eleven is definitely just a starting point.