I only have one email account (former employer) that has been closed but I also have a personalized email address that I own so that I never loose it.  Google may go away but I still own my email.  I use that account to aggregate all my other accounts.  I believe it monitors 4 other accounts so I never have to check them.

As has been suggested, make the change before you have to.  It really is no different than changing your physical address.  Notify everybody you can think of, knowing that you'll forget some and have some frustrations at some point.