If it's a posted position, it's Signing specialist (signing specialized team member, blah blah whatever the hell they call it nowadays). They wouldn't post for ad-set signing, since that isn't an official position.
If you apply for and accept the position, you should intend to keep it for a while. It is a complicated, demanding position, and they will have to devote significant resources to training you. Deciding three weeks in that it isn't for you, well... let's just say that's the sort of move that would not bode well for your future employment prospects at the store. So, while you might be able to take it and then decide to step back down to cashier after all, you're gonna be wasting a lot of people's time and money, and you'll likely be stuck at the cashier position for the foreseeable future--why would they risk promoting you again after what you did the first time?
That being said, if you are willing to spend a year or two in that role before moving to GSTL, I personally believe it's the absolute best feeder position for TLs. You will have to manage your own time and workload (for the most part), coordinate with everyone in the store (primarily the plano team, of course), etc. It's a position with a lot of responsibility and plenty of opportunity to show that you can handle being held accountable for your specific, individual performance, and it's actually a position where you can largely control your own results (as opposed to, say, cashiering, where you really can't help it if everyone you check out already has a RedCard, or you get a whole week of change-counting grannies and your speed score dips, etc.).