Personally I prefer to have email history online in the form of a local IMAP server like Dovecot. It is saved in maildir format where each email is a separate file.
I can then use an email client to move emails and folders between accounts.
Make sure to have backups. I assume you currently don't have a backup, if it sits in gmail.
I can then use an email client to move emails and folders between accounts.
Make sure to have backups. I assume you currently don't have a backup, if it sits in gmail.