I once heard that their email will remain active for a year, but it was just hearsay.
I would try to update as many email newsletters as you can. Go through your messages and make a list.
Another thing you could do in the interim is set up forwarding on your old account, to go to your new account.
Many people choose to get a separate account (gmail, yahoo, etc) so that you don't lose your email address next time if you switch providers. MTS Windows Live is the same as a standalone Hotmail/windows live account, except if you cancel, you lose the MTS email address.
Also, setting up multiple MTS emails is a hassle. People often don't want to use the primary address because it is usually something goofy that the sales rep chose. And the rest of the process is frustrating. You have to complete the primary setup in order to create a second address (log in, change temp password, input name/address/security question, try picking second address, provided it is not take already!) THEN repeat this process to complete setup of the second address.
Add in elderly people and slow computers and you understand why I don't like setting up my customers email.
Don't forget to check your new account's junk folder sometimes, as some emails get flagged as spam/junk, even if they aren't. Then, mark the messages as not junk, to add to safe list.
Hope that helps