I think that depends a lot on how it's marketed. If it's touted as something that regular users ought to have access to as well, it might tick off a lot of people, but it doesn't have to be like that.
Although I wouldn't be surprised if the knuckleheads at Twitter botched it.
I agree. Think of the way Reddit Gold works. It doesn't really take anything away from the Reddit experience, but it does give folks some additional value that those that feel like paying for it, do. If Twitter just adds features for the premium side, and doesn't strip current features from free users, it should be easy to make that transition.
Although I wouldn't be surprised if the knuckleheads at Twitter botched it.