Thanks for your time @marcdw, it was a simple and successful matter after all. I started it more complicated than necessary.
I copied the new original microG apk (Services Core - com.google.android.gms) to the directory /system/priv-app/GmsCore/.
Then I deleted the /e/ version GmsCore.apk (0.2.25.223616-9 (13f943e)-noen) and renamed the new original microG apk (version 0.2.27.223616) to GmsCore.apk and yes …
… this is probably elementary - gave it permission 644 (rw-r-r).
Although TWRP 3.7.0_9-0 showed red error messages, it finally executed the commands.
Dalvik and cache I’ve not wiped. Whether microG works with all apps used - the question remains to be answered definitively. In any case, all apps relevant for me work.