I have a Verizion Thunderbolt, smart phone and the family plan expires in July. We are thinking about changing to Sprint.

I upgraded my phone last year to thunderbolt while other member used the same phone.

If the plan expires, can i still use the Thunderbolt for like one more year since i upgraded mine and keep the same family plan/ pay the same amount?
Or will the plan change to individual plan since rest of the family is not using the verizon phone anymore? if it chances to individual plan on verizon, I will most likely pay more on bills.