i checked my upgrades for my iphone 3G which i purchased in November of 08, and there was an option for the iphone 2G. Although it said out of stock it also stated: Messaging
Text Messaging
Multimedia Messaging (MMS)
Email
The cost of the package is the same so I imagine its easier to switch all the phones over rather than just the 3G ones. Maybe it will be something internal in the firmware that will prevent you from doing MMS.
Edit: Oops I thought you meant a 2G on your account was listing as having a MMS package.