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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Having had the 360 for a few days now, the minibox has lost channels on movies and sports...
The same engineer who did the v6 install did the 360 and while he was here my partner mentioned why we were being upgraded again and he said the loss of HD channels may have been due to the co-ax connector thingy.