[SOLVED] Nextcloud vs. -current = broken after PHP upgrade

I upgraded -current on my NextCloud test server this morning, and of course immediately got an error. It seems that NC17 will not work with PHP > 7.3 and I now have 7.4 installed. NC18 should fix that, but the stable version of that won't be available until mid-January at the earliest, so I need to revert PHP back to the last 7.3 build, but have never done that before. What is the easiest way to get Slackpkg to back up to the older package? I suspect I need to shut down Apache, uninstall the 7.4 package and then manually install whatever was the last 7.3 build. But then how do I prevent it from automagically upgrading later, before I am ready for it?