I will bet that Comcast switching to Yahoo as a mail provider is intended as a two step process. First invite everyone to voluntarily switch, tell them about all the benefits, etc. Then a year or so later tell everyone else they must switch, since Comcast’s own email system is going to be shut down on future date X. Where that future date is then only a couple months in the future. Then on date X, ready or not, they shut it off regardless of how many customers they strand.
Just like the policeman’s rule of thumb for interactions with the public: “Ask, tell, make.” First ask for cooperation nicely, then order them to cooperate, then apply violence until they cooperate. Comcast is doing the same thing to their subscribers. Right now is “ask”.
Once you get your current email config problems solved, you might want to consider taking them up on that Yahoo transition offer; waiting for “tell” or “make” is usually not the smart plan.