Coming from a Comcast installer that has worked in 5 different Regions over 3years, I remember when a customer was able to tip you 20bucks cash to hook up 5tvs instead of 1 and that was that, just lines. Well FFD to aprox 1.5yrs ago (at least over here in the north east) Comcast has upgraded to an all digital network and rebranded as Xfinity. What this means in english is lets say you were paying for standard cable previously at 67.70/month. thats line off the wall into the tv, auto scan 70-whatever channels. Comcast basically ran two networks at the sametime for quite a while to try to make this as easy as possible on both installers and customers. You should have received mail or notices or whatnot that this was happening quite sometime before the cut off (which has not happened to you yet based on your post) The analog system which is what you previously had, and the digital system which the package is called Digital Starter and costs 67.70/month. So the price didnt go up at all. HOWEVER..... ALL of comcasts pricing is for 1TV with basic services, unless you bundle or promo. Anyhow, for the same price (off promo) you are entitled to digital starter, which gives your a cool box, on screen guide, on demand, etc. Now I know your talking about limited basic, Ill get to that in just a second. All additional tvs on DStarter or above require those small DTAs (Digital Transaction Adapter) you are talking about. Which channels the box decodes is up to the computer system which provisions the box based on which package you subscribe to. Now back to the analog system, Comcast will slowly (they are finished in my home area) begin removing channels from the analog frequencies in order to reassign newer/better/whatever you wanna call it digital content to the frequency that analog cable used to operate on. Keeping in mind with digital and frequencys, were not talking about audio and video anymore were talking about data, kinda like how hd is compressed and MPG4 is the new stuff that replaced MPG2, same idea, the company changes internet services around, adds phone features, etc. So by *Forcing* all customers to Upgrade to digital something, they are able to make vast improvements on their network that will benefit everyone in the long run. Ok so thats basically how and why its happening, sorry for rambling... heres the answer to your question
The channels you are receiving that you refer to as "free and clear" via comcast are more-so bleeding through the line, because your tv has a qam tuner in it. (basically it has a generic digital box built in) so you are able to pick up select random channels, and whatever analog is left. I guarantee you that all your locals will be gone within a year, maybe 6months, maybe 3, who knows. Now, you mentioned your hd locals, they said you need to pay extra for the hd box. Well yea thats true, but there is another way. Local channels are broadcast openly. With the correct splitter and a trip to radioshack/walmart you can keep them without paying, and your not stealing them.
run the cable off the wall into an "Antronix" branded 2way splitter, using anything bought in a store can greatly hurt your signal. run one line directly into your tv, autoscan, pick up your locals and whatever else you can. run the other wire into the dta. Unfortunately from here you will need to swap the output of the dta with the direct line when you want to watch one setup or the other, but it is possible. any questions let me know.