How do you set a plugin version as "changing" in the Plugins DSL, so that Gradle re-evaluates it even if the version number hasn't increased?

I didn't think you can. The easiest is, to make the plugin use a

-SNAPSHOT

version as long as things are changing. Other than that, you probably have to switch to the legacy way by adding the plugin dependency to the classpath configuration in the buildscript block and there the code artifact, not the marker artifact. There you can declare it to be changing.