I had no trouble with transferring the TimePort on my 2002 CLK 55 to my existing Verizon account last year (of course, this is a later model.) Activation took some doing (I tried first to use the web based interface), but after several support calls it worked fine. The TimePort will not update the firmware in the same way new phones do.
As you know, the cell phone type has to match the system of the provider. For example, Verizon uses CDMA. It is also likely that they will require digital capability, since analog is being phased out. Although I am not familiar with the Mercedes specific version of StarTac, I know that they were generally available with both analog & digital capabilities.
Phone stores might be reluctant to help you, since their commission might be tied to selling phones. But you could always try getting a new account with the minimum phone, and then try switching using the telephone support line.
-- Harald
