First time flying to Washington (IAD) for an upcoming weekend. Booked a UA codeshare on an AC flight and noticed that I got seat assignments when I wasn't expecting to (United T fare) as I was going to book seats because the seatmap was already pretty full.

However, if I go to the ancillary seat selection site, it would charge me to switch seats to another regular seat. So, I'm not sure if our seat assignments will eventually get rejected at some point or something. 016 ticket number issued, and checking the booking on AC.com shows the seat assignments.

Should I just pay to switch seats to make sure? It's only $10 each so not a big deal.