There is a window to opt out of the change, but if you don't expect to pay more for a new plan.
T-Mobile Is Forcibly Moving Those on Older Plans to Some of Its Newer Ones in November. Key takeaway: You can Opt-out but have to call T-Mobile Customer Care to do so. My Magenta to Go5G basic plan would cost $240 more annually for the added additional bonus of...extra hotspot data. That's it.
Honestly this will probably be what makes me transition to Visible. I'm currently on a $100 for two lines Magenta Max plan. I know I can opt out this time, but the grandfathered pricing is the only reason I'm still with them.
I'd suggest US Mobile if you wanna choose between T-Mobile or Verizon. They can use either/or pending on your SIM card. I pay $90 after tax for 2 lines. Both unlimited everything and 1 with 50GB hotspot and limited international.
I've been using Visible as a second SIM for the last year and I haven't had any trouble. We were RVing so I was bouncing between T-Mobile and Visible depending on who has the better coverage.
Edit: and I can easily find people reporting the same issue on US Mobile