Hello

your emails downloaded via POP3 are on your computer and they are not going away (unless you don't backup your system and your disk dies of course). About your 'save via Imap', be wary of this, with Imap the folder that appears in Thunderbird is a cache and may not include the full email for all messages. In your place I'd check that the account sync parameter are on 'sync all messages', repair the Thunderbird folder, install the exportexporttoolng add-on and do an export of the folder. If you want to be really paranoid, import the result in Local Folders, check the folder size and read a few old mails with attachments.