You can install Thunderbird without losing any existing messages and settings.

So wrong.

The new version found nothing from my existing installation and asked for settings from scratch. It also could not verify my Gmail mailbox.

So I uninstalled the new version and rely on the older one. Unfortunately, because of the attempted installation, I cannot make it the default email program and now must use on a hybrid model -- Thunderbird for certain functions, and Windows mail for sending articles from online publications.

Don't give up yet. It's not unusual for a new installation to prompt for email id. This happens because TB often doesn't see the existing account profile. When it prompts for email id, ignore it and click the 'home' tab. If you're using Windows, right-click near top of panel and activate 'menu bar' and then click help>moretroubleshootinginformation, scroll down to Profiles, click 'about:profiles' and it will probably show your old profile.

" it will probably show your old profile. "

Probably is not nearly good enough. Although TB is installed on E:, the profiles are stored on C:. I did find the details via Tools/Account Settings.

"TB often doesn't see the existing account profile. When it prompts for email id, ignore it and click the 'home' tab." Who knew?

The original popup window stated that my TB version could not be upgraded, which was a first. But, based upon your assurance, I went for it. All of my previous, automatic upgrades were seamless, perfect. This download was not, so that tells me the new version is defective or deficient in this regard, not ready for prime time.

Today, I was able to restore TB as my default email program, so I'm back to business as usual. FYI, I began using Eudora in the early 90s.

If you still have Eudora mail in your profile, corruption is almost certain somewhere in the store. Eudora was very good at being corruption tolerant. Thunderbird very much not so much.

The issues with upgrades not being able to complete, is almost certainly related to third party software you have running on your computer. Antivirus products and problems with Windows UAC are both common issues and well as upgrading to a 64bit version from an existing 32 bit one causing a new profile. This is separate to the upgrade failing. (Think scanning and locked files as reasons for updates failing).

This latter is one of the more common causes of profile loss as Thunderbird identifies the 64bit version as a "new" version for the version per install "feature" as Windows installs it in a different folder. I don't consider it a feature. The developers do. See https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation

Back in 2019 I wrote a blog post explaining how to recover from this nice new feature https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

However your issues are not bugs, nor do they indicate anything but you have now probably moved to a 64 bit version of Thunderbird.