I can't speak for Thunderbird, but the last time I built an application that used email protocols, I used libraries that provided separate POP, IMAP and SMTP client modules. So you would be using entirely separate code for POP vs IMAP and therefore it would not be feasible to effect a changeover by simply adjusting stored parameters. The advice on these forms is always to create a new account and not attempt to repurpose an existing account. There is a tool provided whereby you can set up a configuration script that Thunderbird would refer to when setting up. https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration This would ease the pain of setting up each user individually. You might also use a local script to copy message store files into each user's Local Folders, allowing the existing POP-based account to be deleted. You cannot just paste these into the folder system of the new IMAP version of the account, because in the case of inconsistency between client and server, the server wins and those files would be silently removed. You may be able to drag and drop folder systems from Local Folders to the new account's folders in Thunderbird, saving you the effort of manually recreating users' folder structures, but I have to say that it is very easy to set up moves of very large blocks of data, and email protocols don't handle this very well.