POP to IMAP: delete POP account without losing mail?

No, that would not be elegant at all.

Try this, instead:

1. Create a new folder on your Desktop and name it POP Mail.
2. Inside that folder, create another folder, named POP Messages (or whatever you prefer to call it).
3. Place the POP .mbox file inside that inner folder.
4. Use Mail's Import Mailboxes… feature, choose "Other" from the list of options it gives you, then navigate and choose the POP Mail folder; don't go any deeper in the folder hierarchy. Mail should find the .mbox file inside and present a list of mbox files to import, with that one already checked. Choose "Continue" to finish the import process. If successful, Mail will show all the imported messages in a folder named Imported, from which you can then move them to wherever you like.

Greetings,

Is there a way of deleting this account (or at least getting rid of that icon) without losing all the archived mail in my Received Mail folder on my Mac?

Yes. First you need to go into Home/Library/Mail and remove the POP account mail folder; then you can delete the pop account from Mail. If you delete the account first, all your mail in the POP account on your Mac will disappear, never to come back.

Did you mean User/Library/Mail?

Yes, but not everyone knows the path conventions, so I use Home, because it is the same as your user account.