Are you able to do any earlier BT? If not, I'd take that opportunity to throw in a 2 nap day to shift WU later again. Sometimes with the move to one nap, you end up with a 23hr day and need a 2 nap day here and there to get back in synch with the sun, yk?
If you are able to be flexible with BT, you could try for the normal nap time and see how they cope, knowing its a longer A than they're used to so there may be a 30min waking to resettle but that they also may nap for 3hr (delicious when it happens). They may well take a longer nap and still be ready for bed at the normal time.