At the outset, you're definitely using a V6 and haven't chosen to "migrate" to 360? Check here for the remotes (the point of difference) and "TiVo" next to the video preview in your menus?
On the basis that you are... are you saying you still have the SD channels in 510 upwards, but the 50x HD variants are greyed out in your EPG?
Then... follow the "Refresh my channel list" via Home > Help & Settings > Help > TV Care (that it sounds like you may already have), a precautionary reboot via Home > Help & Settings > Clear or restart - can't hurt.
If all these steps fail, there's a very good chance the HD channels aren't on your subscription and you'll be best off calling into get that corrected. It's odd for them to "drop off" without something triggering it.
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