Note: Before removing your account (as described below), be sure to confirm whether any other accounts are configured in Apple Mail. In order to preserve any local mail in the "On My Mac" section in the left-hand sidebar, you will need to have at least one account active in Apple Mail at all times. If you do not have any other accounts added to Apple Mail besides your MIT mailbox, consider adding another mailbox (such as a Gmail account) temporarily: https://support.apple.com/guide/mail/add-email-accounts-mail35803/mac#:~:text=In%20Mail%2C%20choose%20Mail%20%3E%20Add,use%20that%20account%20with%20Mail.
Once your mailbox is successfully migrated to O365, you will need to delete and re-add your account. This will temporarily remove your mailbox from the software. Instructions for doing so can be found here: https://kb.mit.edu/confluence/x/yDtBCQ
If you have a local archive of email files that you have manually added to your Exchange account in Apple Mail, you may need to manually reconnect those local archives again as well (see note in bold above). If you did add an additional account and would like to remove it after adding your MIT mailbox back to Apple Mail, see here for instructions on how to do so: https://support.apple.com/guide/mail/log-out-of-or-remove-email-accounts-mlhl9eb9408c/15.0/mac/13.0