FYI - after another rescan - I have my locals (HD) back.
JagFan28 - I'll see if I can answer some questions, but I am new to Comcast (3 months) so others can jump in and correct or add anything I am about to write.
This is what I know (and don't know) - the box your friend is refering to is the DTA boxes that will give you all your channels (basic, expanded) in SD
so they can get rid of the analog channels to make room for more digital (HD mostly) channels.
Now I too have limited basic that comes with my internet service and that is all I want right now (mostly locals in HD is my priority) hence the semi panic the past two days.
No one really knows with comcast jax will really do after the transition (DTA) as far as encrypting HD locals and other channels but other markets so far have been pretty lucky and their HD locals have be left alone at least from what I have read.
As bubba mentioned above the channels are now on a different frequency and the filters that Comcast use are probably blocking these signals for you. If that is to change I do not know, just keep rescaning and go to this website to check for clear qam channelshttp://www.silicondust.com/hdhomerun/channels_us
As far as the DTA box, not sure about what you are referring to about registering the TV or if you will lose clear QAM but if you are feeding you TV with only the DTA box then yes you will lose clear qam as there is probably only 1 rf (rg6 cable) input into your tv so you either connect the dta or coax for clear qam.
Technically after the DTA transition the only channels someone without a Comcast Box (DTA, DVR or other) will be the limited basic in clear qam and I am not sure that includes the HD locals just the SD versions.
Now as far as OTA with an antenna - it depends on what type of box you get - you can buy one for HD or SD - just research which one you want. I use an old Directv receiver for my HD locals OTA (over the air).