Thunderbird question

[Phil]

Thank you for reading this and while my question may be considered off 
topic I hope someone may be able to provide an answer.

My ISP has changed it's mail server from POP to IMAP. If I delete my POP 
account within Thunderbird that will also delete all of me e-mails as 
well. So my question is, how do I restore my old e-mails to my new account?

A Google search hasn't helped and I have tried copying from my backup 
into the newly created Thunderbird folders without success.

-- 
Regards,
Phil