I don't think there is a way for Comcast to mark a show/movie as watched if the show moves to a different channel or platform. One thing I do is set up a recording for the "ABC" show and then I specifiy the channel, say channel 400. If that show gets moved to channel 356 (meaning they are likely all repeats) then it will no longer record them.
Having said that, the watched/unwatched feature is faily useless. There are lots of complaints online about shows that were marked as "watched" becoming "unwatched" when the box reboots or when there is a half-moon, or whatever. I've given up on it. When I used TiVo this worked seamlessly but those days are long gone.
I wish Comcast would just admit that this fearure is buggy as h e l l and remove it until they can get it right.
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