That would be the expected behavior when you simply changed an existing IMAP account.

Is there any way I can get them back?

Do you still have access to the old account?

Did you turn on 'synchronization' for the old account, assuming it was indeed IMAP?

Do you have a backup?

Thanks. I haven't got a back up. But when I do a search for old emails I get a thread but no contents. Cheers

What you see is only the information in the search index, not the actual message. You'd need to rebuild the index to get rid of the orphan information. https://support.mozilla.org/en-US/kb/rebuilding-global-database

Thanks