How do I change my POP setting to IMAP without clearing my already downloaded email ?

I've just had a nightmare as a result of following this advice.

I wasn't aware that when I copied the emails from 'On My Mac' back to newly created IMAP account, that the emails were also going to be transferred to server of new Email hosting service which had a MB limit. So when it reached limit it gave me notice that inbox was full. Result: 865 emails copies out of 1400+ emails.

Additionally, I didn't want my emails to be stored on external server. When I realised I had 'uploaded' my old emails to new Email hosting, I tried to copy them back to folder on my Mac.

Result, emails lost on way over (on IMAP) server...as copies were somehow lost because I deleted folder thinking operation was done (it wasn't). So when I got notice that Inbox was full, I had already deleted the folder on my Mac.

When I tried to copy them back...on one inbox it lost all emails (as it hadn't properly ever copied them over before crash.

Plus, I never wanted emails to be 'stored' on external server.....

So beware people that migrate from POP to IMAP. Emails are not stored only on your computer once you download them.

On IMAP what is 'downloaded' on your computer still stays on server. If you delete it from server it's deleted from your inbox as well. You retain your own copy only if you save it to own folder from inbox. UGH

Create some folders under "On My Mac" and move your present emails to those. Then delete the POP account, make the IMAP account then you can move those emails back to the new folders. or you could export the email folders to your desktop and import back later. I normally do the "on my Mac" process.

You may be able to retrieve the "on my Mac" mail. Open Finder, then using the option key click on GO then library. Go to mail/V8 (or perhaps V9) folder. There you will find folders that have cryptic names. Open those and look for the folders from "on you Mac". If they are there you can export to your desktop and then import from mail to restore. Also. did you make a TM backup before moving the email files? If you did you may be able to also retrieve from that.

Correction: When you find the email folders in your Mail directory you need to copy and past to your desktop (not export) then you can import into mail.