Xbox Gamertag Change/Availability

"How to change your Xbox gamertag"

Here's a link to a website that literally checks if a GamerTag is even available on Xbox in the first place!

"How to tell if a xbox gamertag is available

With the new system it will definitely be faster if not almost instant (24-48hrs most likely) for an unused gamertag that once changed to something else (thus making it unused) to be free for the community pickings once more. The new gamertag update was
supposed to allow for all players to have the gamertag they wanted no matter how common or uncommon it may be, with it also brought changes to most likely the systems of how gamertags are viewed in Xbox's system as a whole. You should still take that with
a grain of salt though, as instant may not necessarily be true but it definitely won't be years down the line when you can use that gamertag again... I'd think more like days or weeks at the latest.

Yes both literally and theoretically. Literally because after a certain amount of time it will be useable by any other account again once the change on the account for a new tag takes hold... and then theoretically because if you were trying to use that
original gamertag again, that waiting period could either make or break you. If its a less common gamertag/combination of words and phrases then you'll probably be safe and able to get it back no problem. If its a more common and highly sought after name or
word combination, then after the period where if becomes available again... it wouldn't be surprising if it gets taken by someone else.

yes but leave it at least 48 hours so it can go back in the system as being Available

Yes it will but it's already in the pool just the original isn't in there yet but other people can use you're name now it will just show up with tags like#2999 for example