This article will outline how to switch an account from POP to IMAP. Note that you cannot convert an existing account from POP to IMAP. The account must setup as IMAP along side the POP version, and then content must be copied from the POP account to the IMAP account.
How to Convert a POP Account to IMAP
Go to the File menu and select New > Mail Account. Enter your name, email address, and password, then use the automated suggestions for IMAP, or enter the correct IMAP settings in the account setup window.
Your POP Inbox may have more messages than your IMAP Inbox, depending on how you had the POP account setup in Postbox.
Your POP Sent folder may have more messages than your IMAP Sent folder, because only POP Inboxes sync with the server, not Sent folders.
You will have storage folders in your POP account that are not in your IMAP account, because only POP Inboxes sync with the server, not storage folders.
If you see a message in your POP inbox that is not within your IMAP Inbox, then right-click the message, select "Copy To..." and then navigate to the IMAP Inbox.