I didn't realize until yesterday, but in-between the early episodes I've seen on the TV Land primetime line-up and the later episodes that were shown on the Fandemonium marathon, the Cleavers are in a completely different house! Everything's different, the foyer, the kitchen, everything! In which episode (if any), did this move take place, and is it ever explained on the show?

'The move' was not shown in one ep, but it took place between the 2nd and 3rd seasons. In several of the later eps of season #2 they are talking about selling the house and looking at other houses; and finally there is one where Ward and June come to the guys' room to tell them they have sold the house. If I recall, in the opening ep of season #3, everything seems to have been dealt with about the move, but Wally finds himself continually being where a conversation he doesn't want to hear is going on, so he says, "I was just going to the den;" "I was just going to the living room" "... the kithcen"... he finally says, "Boy, there sure are a lot of places you can go in this house."

The reason they had to change sets ("houses") was something to do with change of studios or the avalability of certain portions available for filming. But in the series you can put some idea together to see that the Cleavers did not move very far. For example, when Gilbert moves to Mayfield he lives right across the street from Beaver. Later, in "Beaver Takes a Drive," it is said that Gilbert's house is on the next block.