You don't.

You create a new account in Thunderbird, but select the required server type at setup. Thunderbird will usually default to IMAP if left to itself, but with some providers it stops and offers you the choice. In some cases, it can only find one type of account and if you need the alternative type, you need to use the "manual configuration" button enter the values by hand.

Thunderbird should, and normally does, allow you to set up multiple accounts at the same server. I have run POP and IMAP connected variants of the same account side by side. However some users experience a problem where Thunderbird refuses to add a second account because "this server already exists". If you encounter this, I'd suggest you stop, move all the messages from the old POP-connected account into Local Folders, delete the POP-connected account and start over, adding the new IMAP-connected account.

(When I last developed a program that used email connectivity, I had an environment where there were SMTP, POP and IMAP client modules that you just dropped into your own program. There are enough differences between these, I think, that you can't simply swap one for another. There are fundamental differences in the protocols, the message storage systems, the options to be made visible to the user, such that you can't just swap one client module for another. I don't think I have yet met an email client where you could retrospectively change from POP to IMAP or vice versa without effectively creating an new account for the alternative protocol. I've seen advice on places like gaming forums to simply change the port number, but I can't say I have any great respect for such suggestions, given that most of the advice bandied about on those forums reveals a lot of ignorance and a great many half-educated guesses. Gamers in particular tend to be risk-taking personalities, what with overclocking and the like; their interests are in cutting-edge performance, not stability and reliability. In Thunderbird, at least, there are settings specific to IMAP and others specific to POP; if you were to switch an account between the two, would it be able to also switch the user interface to offer the appropriate set of settings?)