I just wanted to comment regarding the rule you mentioned and wanted to say that I was always told that, unofficially, you can always move up but only a tier at a time and you don’t want to move down if you can avoid it because then you likely have to move back up. The calculus does get harder though the longer you’re in the game — you might elect to move down because you’re significant other gets a promotion or you don’t want to uproot your family, etc.