I got that mailer too. It didn't say one thing about about QAM.
And it never will, even mentioning "clear QAM" is a nightmare for the Comcast support folks so they just ignore it. There's no upside: their only goal is to help you rent more of their equipment, use their video on demand services and bundle phone with video and hi-speed internet.
Pretty sure the "all digital" means just that, they are doing away with their analog offerings, not clearquam which is already digital.
It's what comes after
that really matters, hence the hint of fear in FredT's post...what Comcast (and every other cableco) really wants is all of those digital clearQAM limited basic channels to be encrypted, to be locked down requiring additional equipment. That would mean a current HDTV with a built-in QAM tuner or an EyeTV clear QAM tuner like the HD Homerun FredT mentioned, would no longer be able to record the "must carry" locals in HD just by plugging the bare coax in. Most of us view that depressing development as an eventuality. (See FCC filing 11-169 if you're interested in this...)