In Thunderbird, click on the account your want to change. Then select Edit->Account Settings. It should bring up a dialog box like the following.
At the bottom left of the dialog box you can click the "Account Actions" button. Select "Add Mail Account". Fill in the fields with your information and click 'Continue'.
After a period of time, Thunderbird will give up trying to find the proper server - since all mail is handled by Microsoft there is no wsu.edu server any more. We have to set things manually. Make the resulting box look something like this, changing account names as appropriate.
Click 'Re-test' to double-check your settings. Then click 'Done'. Now click this account at the left of your Thunderbird screen. This is where you will read your mail in the future. Any local folders you use remain valid.