I've had a similar experience today. Not due to the nasty seat shifters or an equipment change just trying to move to an exit row.

It was fine on the outward leg but wouldn't let me change the return.

After being on the phone to EC for 20 minutes I managed to get it changed after numerous conversations such as being told I couldn't change it because my seating preference was aisle.

Anyway it's sorted now but could be so much painless if the "bug" was fixed!

Cheers B