I've recently decided to transition all of my personal email from one service to another (Comcast to Gmail). It's been a slow process to change all of my email newsletters, notices, etc. over to my Gmail account, but I'm slowly getting there.
I have the luxury of taking my time with it, since I'm still able to access my Comcast email and have not closed that account. I've found that my Roboform password manager software was a good place to start with a list of accounts that used my old Comcast email. I'm slowly converting those all to use my Gmail account instead.
I'm also monitoring my Comcast email as it arrives, and either unsubscribing or changing each one over to Gmail. I've found that certain providers will let me update my email address, but they don't seem to automatically remove the old email from their list, so I've gotten a few duplicates. If I get a duplicate, I just unsubscribe on the one sent to my old Comcast email so that takes care of it.
As I finish this process, I'll post any other things that I discover.