Yes, if you live in an MPEG 4 market and cancel HD Access they will lock your receivers down to Hide HD duplicates (so only SD channels show in the guide), and de-authorize all of the HD channels except for your locals (so your existing series links won't work anymore). I believe they also lock the box to a maximum of 480p, but I'm not positive about that, you might still be able to watch your locals in HD instead of downscaled to SD.
Why would they "lock" the box down to 480p when you can get your locals OTA in HD with an AM-21 even if you AREN'T in an MPEG4 local market? I verified this when a friend borrowed me his AM-21 and I hooked it up to my R22. I was able to record the locals in HD and play them back at 1080i.