Recently due to some issue with my current hosting provider I changed the same. As the mails configured as IMAP(Local Download) i was not concerned about the mails.
To my surprise on entering the login details of new hosting server the mails from new account starting downloading but previous ones got vanished. I can see the dat files more than regular size in the Emclient folder(Roaming Profile) but still not showing the old mails.
Tried re-installing and over-writing the dat files still no mails showing.
Normally If you remove and IMAP acct mail which has mail folders associated with them, “unless you have previously dragged them” into the (local folders at the bottom left) they would normally all vanish and would be replaced with your new ISP IMAP mailbox folders due to IMAP mail is all remote.
The only way you could then restore them is to either re-add your prev IMAP mailbox account (as a second account) and then once added, drag those prev emails into your new ISP IMAP mail folders. Then finally once the prev old mail are all synched / copied to the new IMAP mail folders, remove the old acct.
Alt restore your eM Client prev backup if you did one via “Menu / File / Restore”. Restoring an eM Client backup would of course reinstate only your old ISP IMAP Mailbox account & folders. You would then add your new ISP IMAP acct & drag / sync the old mail across to the new mail acct folders etc. Then finally once the prev old mail are all synched / copied to the new IMAP mail folders, remove the old acct.