It's my understanding that even if you buy a phone at full price now and switch to 4G, when you then use your upgrade later for the Fighter you'll be forced into one of the tiered data plans.
I'm in the same boat. My contract isn't up until October but Verizon has offered me an early upgrade. I'm hoping the Fighter comes out before they force me into a tiered data plan. I've also thought of buying a used 4G phone now and then buy the Fighter at full price when it's released. Until more information comes I'm just sitting in a holding pattern.