First I was using this for reference.
When changing from POP3 to IMAP, its better to delete the account and create a new one. Thats because I don't think I you can change the "Account Type" under File > Account Settings > Account Settings > Change from POP3 to IMAP. That is only done during the creation of the account. Changing the incoming server only changes where its trying to communicate, not how it is trying to communicate. POP and IMAP are two separate protocols , almost think of them as different languages.
As a secondary issue, it seems Outlook defaults to IMAP if it detects the server for that domain. And since you can't go back and change a IMAP to a POP, its better to choose "Manually configure server settings.." when adding a new account. That way you can even choose where you download your data file so you can change email providers but keep the same inbox and rules.