Boxmaker, I use Comcast as my ISP, and also use the email they provide. However I also have an AOL email and a Gmail account and use them for different purposes. You can go to AOL anytime just like any other web site by typing in AOL.com, and still sign in, read and send email. Same with Gmail. You'll need to go back to your old email client from time to time to see if anyone has been trying to reach you with the old address. On the plus side, changing sure stops a lot of spam, etc. for awhile!
Also, many of the email services will look at your address book and send a notice to each and every one of the addresses that you have changed to whatever new service you've chosen. It's not automatic, but if I remember right it gives you the option to accept or decline. Also imports all your old addresses into the new service.
No, you don't have to be invited to any of them. Just type in the web address, go to the site, and sign up.