Most CSRs I have talked to, either through email or phone, have all offered to cancel my order if I wished to. Maybe you just need to get the right CSR when you call.
When is your ship by date? If it is close to the release of the Droid X, you could always accept the shipment and then immediately take it to a Verizon store and switch it there. If you wait and reject the shipment, you will have to wait a few days for your account to be updated before you can get the Droid X. The rejected package will have to be processed once it is returned back to Verizon. This is assuming you are getting a phone through NE2 or something like that.