It should be as simple as adding the new account and setting it as the default in Thunderbird. The old account will stop working, anyway.

Have you considered using one of the free email providers, independent of your ISP so you'd never have to do this again? You'd also have the benefit of always being able to work with your email when away from home.

You should also give some thought as to what you need to do about your present email account. If it uses IMAP then you will lose access to all your stored messages when you switch providers. If it uses POP then you'll keep your messages but will get irritating error messages if or when it tries and fails to connect to the server. In both cases, the answer is to move all the messages you wish to keep into Thunderbird's Local Folders account.

Zenos, thank you for your message! It is helpful.

In fact, I was trying to find where in Thunderbird the default button is hiding. I want to default to another established email account - one of five I have operating inside Thunderbird before the one hosted by my current server goes dead. I can't find the 'light switch!' Where in T'bird is the switch for naming an account "default?" Please be specific, as I have looked everywhere I know to no avail. I don't want to delete the current default account in T'bird until the switch is complete, so I can notify all my contacts who only have my default account as their means to reach me.

Can you find your way to Account Settings?

At the very bottom is a button which is usually labelled Account Actions. This button provides a drop-down menu which lets you set the default account, among other things.

The Account Actions button has a remarkably silly design in that it changes its label according to what you last did with it, potentially making it impossible to definitively describe and identify.

THANK YOU!!! I found the 'default button' right where to said it was! You're the MAN!