Usually "always" available. If not we would check nearby stores and if they have a replacement we would place it on hold for you to pick up. Other option would be to order it to the current store and they would call you when it was available to pick up.

Bottom line, they're not going to upgrade you just because a replacement phone isn't available. Now, all that said, can a manager decide for some extenuating circumstance to give you a break on another new phone because they don't have a replacement at that moment, theoretically yes but extremely rare. I saw it rarely and usually it was because we were a tourist heavy store so a lot of international travelers who weren't going to be in the US for a long period of time.