No outgoing mail after Bell migration

I use Apple Mail on an iMac with OS El Capitan v10.11.6. I've just had to migrate my Bell sympatico email account to Bell's new platform, after it ended its association with outlook. I followed all the steps Bell recommended, and have spoken with Bell twice, but I still cannot get the outgoing mail account to work. I receive mail well enough.

Initially, I was able to trick the system by manually typing in the password for the outgoing server under Mail/Preferences. It would work for an hour or two, before shutting down again. Now even that doesn't work.

Bell tells me the problem is because Mail is confused between the new IMAP sympatico account and the old POP sympatico account. In order to resolve the problem, I need to delete the old POP account, BUT, I would likely loose all my folders on Mail. Needless to say, that is not really an option, since they contain years of communications and cannot be reconstructed.

Is there a way to save all the Mail folders and content, and then reintegrate them with the IMAP account once the POP account has been deleted?

Is there another/better way to handle this?

Thanks in advance for any guidance.