This one once upon awhile was the other but this is the other one nighadays

this one once upon a time was the other but this is the other one nowadays: this appears to be saying that room number one used to be room number two but the numbers were swapped, but given what happens in the Night Lessons chapter (II.ii) my first thought was that we are being told Shem used to be Shaun and Shaun used to be Shem but they have now swapped names. Or it could be that the boys used to sleep in the room that is now Issy's.