I had an ATT unlimited data plan, grandfathered from way back in 2004 or something. It was wonderful while cellular data was still only available in limited amounts. I only finally moved off it about 3 years ago. ATT never seemed to care much. The only problems I had were when I had to make certain changes to my account, the staff could never find my plan in the system -- on more than one occasion we had actual ATT database admins on the line to resolve things.
I had the exact same situation. I stuck with it through the limited data plan era with my grandfathered ATT plan and only changed it once we got the unlimited data plans again. It was great, the only problem I had was even though the data was unlimited, the grandfathered plan had a 400 text messages limit which wasn’t enough.
Had the same idea from 2004 to 2010 at T-Mobile in Europe before they were bought out by VC. They just canceled the plans and switched me over but due to their special "after 6 years you triple the data and call volume and it never expires" I had booked enough minutes (which were exchangable for data on the new plan) to have free internet for another two. Of course the new contracts all had expiring minutes as soon as they figured that out.
