The question is not answered, because what occurs depends a lot on the server being used and the capabilities of the server software.

So you will need to experiment, and even if it works, you probable need to keep archiving t less that a few hundred email at a time.

(Not for gmail or Google apps accounts) Right click the account in the folder pane. Select settings In the copies and folders on the Account settings Click the archive option and set the defaults for archive storage format. (I prefer to retain the existing structure) Change the keep message archives to Other and set it to use a folder in local folders (Archives would be good)

Select an email in the accounts list and press A Confirm it appears in the local folders archive folder you specified.

Thank you for your response and clarification. Just one follow up question I have archived all of the email in the account (317 - the most important emails) that I want to keep. The structure I have used is the monthly. So the emails are now on my local drive. If I change hosting companies those emails on my local machine will be kept by Thunderbird regardless of the server settings, correct?

So long as your archive is under Local Folders, as Matt advised then, yes, your messages are stored locally, independent of the server.