When I switched my family plan about a year ago (didn't even really switch plans, just added a discount because of my employer at the time) I had to renew my 2 year contract. Basically my contract started again from that day, however it did not affect my upgrade date at all.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775