Originally Posted by findark

Odds are you will be fine. Your frame hasn't been "swapped" in any real sense of the word - it's just now that they have any new 753s flying they want to come up with a "composite" seatmap such that regardless of which frame flies they don't sell seats that don't exist (normally this process makes a lot more sense, like blocking half of row 12 on the composite 739 since it's only sometimes E+ -- this particular composite seatmap is a little crazy).

Right now, there are 21 753s in service with United: 18 are flying in the old configuration, 1 is flying in the new config, and 2 are in the shop for modification. This likely won't change very much in a month, so you probably have about a 3 in 20 chance of seeing the new config.