By your post, we can NOT tell what boxes or package you have now, OR what state the digital transition is in yoru area. Thus we can NOT tell if COMCAST is doing you a favor trying to get you the equipment you'll need soon -OR- if COMCAST is pestering you to hoc services and hardware for additional fees.
Pretty sure all the boxes are digital (internally), its just a matter if the box outputs analog or digital. Any analog channel that still exists, has a digital twin and the boxes just tune digital channels and converts them to analog to ouput to your TV -OR- in the case of an HD STB with HDMI, outputs the data digitally.
At the beginning of the Digital Transition, analog channels will be replaced by digital channels and you can fit a whole lot more digital channels into the avialable bandwidth then you can analog. as well as carry channels in HD. The channels in the standard packages will be clear QAM, which most digital tuners can tune in.
At the end of the Digital Transition, COMCAST will encrypt all channels they legally can, and none of your personal/3rd party/open market equipment will be able to tune those channels. COMCAST is required by law to keep your local channels in the clear, and you will be able to tune them without a box if you have a tuner capable of tuning them.
Also keep in mind, the packages will change, and despite the advertising and fliers leading you to believe otherwise, you'll still keep your old package, which didn't have new features of the new packages, and as an existing customer you won't get those new features, even though your paying the same price as packages with them, you'll have to notice this and contact COMCAST and have them change your package over to get the new features.
Hope that helps, you'll have to post more info for any of us to figure out why they would be calling you to upgrade, and whether its legit or pestering.
e.g. What boxes do you have now, what package you have now, whats the state of the digital transition in your area? i.e. haven't started converting analog to digital channels; converting analog to digital, but digital is NOT encrypted yet; Digital Conversion complete, all channels, legally able, have been encrypted and locked into COMCAST's propietary system that will NOT work with personal/3rd party/open market equipment.