I couldn't do this either. Got kinda close, but....
I have an existing Verizon wireless account. I want to get rid of my
old Motorola and start using the Droid with my existing account/
number. When I called Verizon, the rep, after a few minutes,
recognized the Google program and told me I could transfer the phone
at any time. Note - she had no record of the phone via the phone
number, but we had to dig out the internal ID before she could find
the phone on the network. We immediately tried to do the switch-over,
but the rep discovered that she lacked an authorization required to de-
provision the phone from its initial activation. She suggested I
would need to contact someone familiar enough with the Google IO Droid
Verizon program to deactivate the phone, then we could reactivate it
transferring over my existing number. I never figured out how to
contact the right people, so my new strategy is to wait until the 30
day period expires and then do the switch. (Or find someone at IO who
can help.)
The Verizon person did explain that I could transfer the phone within
my existing account w/o starting a new 2-year program clock, since I'm
not buying a Verizon-subsidized phone, and that I could keep my
existing pricing - only adding the required unlimited data plan. $30
I believe.
Not sure if this helps, but this is my experience so far. I'd also
like more information on how to get this done.