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.