Lightning is bundled with Thunderbird. https://support.mozilla.org/en-US/kb/thunderbird-calendar-integration

So no, your calendar won't stop working due to a missing compatible version.

Why you do use a beta version on a production machine you cannot afford to break is beyond me.

Encouraged by christ1's reply, which said that I would not lose access to my calendar by updating my Thunderbird v. 47.0 to v. 50.0 beta 1, I started the update but stopped immediately when I received a popup warning that said -- "Incompatible Add-ons Found. Some of your add-ons won't work with this update, and will be disabled. As soon as they are made compatible, Thunderbird will update and re-enable these add-ons: Lightning 4.9b2 Nautipolis for Thunderbird 2.0.33.1"

So it appears that the calendar (Lightning 4.9b2) is NOT bundled with Thunderbird, but is an add-on.

It looks like my problem is now solved. christ1 had wondered why I am on the beta channel in the first place. He said "Why you do use a beta version on a production machine you cannot afford to break is beyond me." The answer to that question is that I am new to Thunderbird. I had read how Mozilla is distancing themselves from Thunderbird and I mistakenly thought that the only new versions now available were betas. So I found out how to switch to the release channel by editing channel-prefs.js. Now, my About Thunderbird states that I am on v. 47.0, that Thunderbird is up to date, that I am on the release update channel and I am no longer being prompted to update to v. 50.0 beta 1. My thanks to christ1 for pointing me in the right direction.

It looks like my problem is now solved.

I'm afraid you're not there yet.

So it appears that the calendar (Lightning 4.9b2) is NOT bundled with Thunderbird, but is an add-on.

Please read the article linked above. Lightning is bundled with Thunderbird, and it is still an add-on. The 'Incompatible Add-on' warning may be a bit misleading, but in case of Lightning you can ignore it.

I mistakenly thought that the only new versions now available were betas.

The release version is available at https://www.mozilla.org/thunderbird/all/

I found out how to switch to the release channel by editing channel-prefs.js.

You didn't. You cannot change from beta to release channel on the fly.

Now, my About Thunderbird states that I am on v. 47.0, that Thunderbird is up to date, that I am on the release update channel

47.0 is an outdated beta. Current beta is 50.0b1. Current release version is 45.4.0 - see download link above. Therefore, no, you aren't on the release channel.

Thanks for the clarification. Can I just download the 45.4.0 release version and install it right over my 47.0 beta version?

I'd first create a backup of the Thunderbird profile. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile

Then uninstall the beta and finally install the release version.

To avoid confusing others you may also remove the 'Solution' tag from your earlier post. Thank you.

@christ1 Thanks for the help. I'm all set now.