You need to logon to your current email address webmail account via a browser and first check that the option for IMAP forwarding is available and selected.

In Thunderbird: File > New > Existing mail account or Menu icon > New Message > Exisiting Mail account enter name, email address and password. click on 'continue' button Thunderbird will try to locate configuration. Make sure it selects IMAP and the server settings for imap. Click on 'Done' to create the IMAP account.

re :The instructions in Thunderbird Help advise that it is necessary to obtain a new e-mail address to set up the new IMAP accouint. Never seen or heard of this. So if you can provide the link, I'll check the information.

In the Thunderbird help topic entitled 'Switch from POP to IMAP account' at the final stage it says: +++++++++++++++++++++++++++++++++++++++++++++++++++++ If you are converting your email account to IMAP from POP, you can delete the POP account when you are completely sure that you have moved all your important messages to the IMAP server (or into your Local Folders). You cannot easily undo this action, so make sure that you really don't need the messages stored in this account any more.

   Open your account settings by opening the menu Tools | Account Settings... , then select your POP account.
   Click on the Account actions button at the bottom and select Remove Account. 

If you have created a new email account that uses IMAP, you cannot delete your old POP account without deleting your email address. Before deleting the POP account, you will probably want to inform people who send messages to your old account that they should switch to your new email address, and allow a period of time to pass before deleting the POP account to ensure that you don't miss any messages. ++++++++++++++++++++++++++++++++++++++++ I took the final paragraph to mean that if I followed the instructions, I would delete my e-mail address by default when I deleted the old POP account which I do not want to do. I want to keep the old address but upgrade the Thunderbird account for it to IMAP.

That is simply incorrect. I have many times setup the same account in duplicate in Thunderbird, one POP and one IMAP.

There is some ambiguity over what "email account" means. In this context, it will be the same email account, but using a different connection to negotiate with the server.

Once you have set it up, your email account exists, whether you use it or not. The default "existence" is likely to be in your email provider's website. You have an email address and there is a server provided somewhere that is ready to accept messages for you. Whether or not this account is added to Thunderbird doesn't affect its fundamental existence.

In Thunderbird, we talk about setting up email accounts, but here you are creating an entry to allow Thunderbird to access your account. So you tell it about your email account and where to connect to the server to make use of it. Thunderbird can make that connection using POP, IMAP, or even both at once.

The "final paragraph" is intended, I think, for those who have had an email account with one provider and now wish to switch to a different provider and a new email account (address). It's a warning that if they simply delete the old account in Thunderbird, its contents will vanish. You need to preserve the contents by copying them somewhere else before deleting the account. Actually you don't need to delete the old account, but you will be periodically nagged by Thunderbird complaining of its inability to connect to the server.

Go to File|New|Existing Email Account, add your account again but allow Thunderbird to select "IMAP".

Now copy (select, drag-and-drop or use right-click|Move to) the contents from your old POP-connected account to the new one. I am assuming that you're changing to IMAP because you want access to your messages from more than one device, so will want some, if not all of your accumulated messages visible.

If for some reason you are not able to set up the IMAP-connected version until you have deleted or closed the older POP-connected account, then you can instead store the messages from the POP-based account into Local Folders in Thunderbird, delete the old POP account, then add your email account again but this time let it select an IMAP connection. Then you can move your messages from Local Folders into the new account's folders.