Over the weekend, my kid lost several Nick channels in the 1700 HD range.
They are still available in SD on the lower channels.
First, you DO realize that NOT all of the channels in the 1000+ range, are in fact HD? Those particular channels you're speaking of, have ALWAYS (unfortunately) been in SD, & all those same channels are simply duplicate #'s, of those same ones in the 2-3 digit legacy range, as part of Xfinity's standardized MCLU that's been around for 5+ years: (note below how they explain how HD & SD channels are handled in this lineup!)
New Channel Lineup Frequently Asked Questions - Xfinity
How will you handle SD and HD channels in the channel lineup?
The 1000+ channels will mostly consist of HD channels. When there is not an HD channel available for a certain network, an SD channel will be broadcast.
They are also still available in HD using the Xfinity stream app using the old channel numbers.
They ARE actually there in HD, because Xfinity just recently added them in HD, but ONLY in IP/X1 only...hence why you ARE getting them on the stream (& Roku) platforms, as well as on all X1 boxes.
Unfortunately, when Xfinity adds these HD IP-only feeds, they are now (supposed to anyway) remove those duplicate 1000+ channel listing on all legacy TV boxes, including DTA's & cablecards - which obviously include Tivo's - presumably to avoid the confusion of having some boxes showing those 1000+ channels in SD & others in HD; this way, the ONLY people that actually can view those 1000+ IP-only feeds, ARE the ones with IP/X1 equipment. The issue happening now is...Comcast does seem to be doing this consistently on all their own boxes - but NOT on CC devices! Hence that Tivo's are now having channels appear in the 1000+ range, where it was previously SD-QAM, but now that that same ch# is now IP-only HD, & when you try to tune to them, you get nothing!
You still have the (SD) versions of all these channels on your Tivo's, but now you'll have to view them down on their legacy channel positions.
Is Xfinity dropping linear HD channels and moving them to X1 IP streaming channels? If so, the Tivo will be next to useless soon.
Been talked about here & other boards ad nausea; right now it's more that any newly added HD channels (& even some new SD channels, actually) are being added ONLY in IP/X1; this offers some info:
Comcast IP not QAM?
As far as dropping existing HD channels, again been discussed ad nausea, with lots of people offering lots of opinions...but at this point, there really isn't much that's been nailed down on Xfinity's immediate future...but it's been VERY obvious for some time, it's pretty much most/all an IPTV future.