it wont cancel your line. however, you will need to activate a different phone on your line before you sell the fascinate, because they wont be able to activate it until it has been removed from your account.
then, before you buy a droid x (i'm assuming you're not paying full price for one in the store), be sure to call CS to verify that the phone is not on the lost/stolen list, AND that it isnt attached to an acct that is in collections or has been written off.