T-Mobile is making it harder and harder for customers to hang on to their old, cheap plans

Are you on one of T-Mobile's fancy new Go5G plans? If not, the "Un-carrier" might try every trick in the book to get you to ditch your old plan, and even do the upgrade without your permission if all else fails.
That's basically what happened to Redditor "rozzy78" recently, and worst of all, the disgruntled T-Mo subscriber is being told there's no way to go back to Magenta Max now. The original switch from that plan to Go5G Plus allegedly took place "by accident" when the user contacted customer support to take advantage of a BOGO (buy-one-get-one) deal they had been offered, but it's hard not to view this situation as a ploy on the operator's part to boost the adoption numbers of its latest plans.
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.
Things that are NOT allowed: