The pay decrease will come with your status change. Nothing you can do about it. It wouldn’t be right for a brand new barista to make 27% more than the other baristas without having to do the tasks that the pay increase is for.

Yeah no i disagree with what the others said. I understand the verbal contract, but anyone who has worked any other job before knows both parties are held to the offer letter. If the Mgr goofed, cool, but they need to fix it with a new offer letter that you choose to accept. In the meanwhile your pay rate while training should be for what your current offer letter says. That's what's in writing, it does not matter what was agreed to in person. Call partner resources and chat with them about it (hr for Starbucks, at least in the states. Canada might have something different) check your handbook.

Idk if it’s different in other places, but in my district you cannot become a shift until you’ve been a barista for at least six months.

I think when they have your application in their system they can only accept it for the position you applied for, so if your application was for the supervisor position that’s what the manager had in their system for you to accept to hired, they can’t change that until after you accept it. Then they would have to change it after. If she told you there were only barista positions available and you agreed to that, then that’s the position you have and will be paid for as long as you’re there. If you do transfer to another store for the shift supervisor position you will likely have to interview with that store manager again for that position. You agreed to being a barista. To get a promotion you always have to interview for the position even if you’ve been a barista for years, and even if you’ve had previous interviews for that same position. Sbux has been doing all sorts of scummy things as of lately so don’t let your manager mislead you, they tend to do things like that.