Downloading Gmail through IMAP

Hello,

I've a Gmail account that has more then 80K emails stored in inbox, I have a plan to now download all the emails through IMAP setting and download all the emails to Apple Mail. likewise, I will be then removing the Gmail account from the Apple Mail and after that will add POP server setting of my custom domain name so we can receive and send emails using that services.

This method seems fine to me, but I have a question that I'm afraid of the prompt Apple Mail ask after removing the Gmail Setting, it shows a message to "re-build the Apple Mail" and so on, the moment I pressed the button "Re-build" it removed my all emails,

So my question is that how do I safety remove the Gmail Setting, so that it should not wipe off my emails, the backup is important to us, afterward, my plan is to use the POP account of my custom domain name.