Cell phone help!!! (i have verizon)?

i have a 2 yr contract with verizon, i was wondering if that means i have to have the same phone for 2yrs or if it just means i have to stay with them for 2 years.. because i have the enV2 and i am much to ruff with it and i want a more sturdy phone like the boulder am i able to upgrade my phone? will theere be any fees besides paying for the new phone? also i want to keep my number