Simple guide to switching from POP to IMAP in Mac Mail
In very basic terms how do I achieve this in MacOS Monterey?
Earlier Mac models
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In Mail Settings, select Accounts. Do not delete the POP account yet. Create a new IMAP account from the same provider. Look on their website for the address of the IMAP server and other pertinent details like the port number.
As soon as the account is set you will start to receive all messages that are on your provider's mail server. Most of them will also be in your POP mailbox; they are copies of what is (or was) in your provider's mail server, so if you see them in both the POP and IMAP mailboxes you can safely delete them from the POP mailbox (or not, you can also just leave them there but don't use them anymore ).
If you want to keep the remaining messages from your POP mailbox transfer them to folders created in your iCloud mailbox or locally on your computer ("On My Mac").
If all works well just disable the POP account (uncheck the box Activate this account) and leave it for a while. Later once you're absolutely certain that all your old messages are safe that the IMAP version works well you can delete the POP account for good.