new address requires purchase of internet gateway...?

Moving my UVerse internet service.  Same service, same speed, same options, different address.  ATT says my existing equipment is "incompatible" and that I have to purchase an "internet gateway" for $100.  However, they can't give me a technical reason for this required purchase, which is tantamount to asking for $100 "because I need it."

If I have to pay $100 for a new piece of hardware, fine, if it's required I don't have a problem with that.  When ATT can't tell me why, that's when I become concerned.  The person who put the move order in said tech support could tell me why, and after a number of "I understand your concerns" and "I realize how important this is to you" iterations, they referred me to sales, which seems to be an ongoing theme with ATT - tire them out before they can get what they need.

Is this a media issue?  Is one neighborhood served UVerse via ethernet, another by fiber, and another by coax?  Is new hardware required as a result of a different media type?  Is this really that difficult a question to answer?  Anyone have any information to share that Kumar couldn't?

Again, this is internet only, no TV (I will not buy TV from you ever, ATT - stop sending me promotions!).