Part of the reason for the plane swaps lately has been the number of p.s. birds in the shop for extended maintenance trips lately. For the past several months there have been at least 2 birds in maintenance at all times with at one point I believe a peak of 4 birds in.
It could be worse though. N529UA which is one of the old domestic 752s without wifi, etc is scheduled to fly flight 414 today from SFO to EWR.