The suspicion is backed by numerous commenters on rozzy78’s Reddit thread, a few of whom are either “verified T-Mobile employees” or purported T-Mo reps. These people suggest changes have recently been made in the carrier’s internal systems to make it even harder than before to switch users back and forth between new and old plans.
One especially trustworthy and seemingly well-informed Redditor claims that Magenta plans have “basically been officially grandfathered” yesterday, which means that intentional or “accidental” upgrades from those options to newer and often costlier Go5G plans can no longer be reversed by anyone but “very high up” T-Mobile employees.
For what it’s worth, this particular user saw no pricing increase when their plan was “upgraded” without consent, which obviously still doesn’t resolve the issue on principle. And if you don’t care that much about principles, keep in mind that you might not be so lucky if this type of “accident” happens to you in the future.
As such, you should be extremely careful about anything you ask customer support to do for you, and if you are thinking of making the jump from a Magenta to a Go5G Plus or Go5G Next plan, try to get all the information beforehand and understand changing your mind will not be an option once you go through with the switch.