Comment # 1 on bug 904351 from Johannes Meixner

According to /usr/share/doc/packages/rpm/manual/triggers
in the new RPM only "new-%posttrans" would run
after all stuff of the old package was done
so that a posttrans scriptlet in the new package
could be theorectically used to fix or clean up stuff
at the very end of a package update.

But as far as I know %pretrans and %posttrans scriptlets
are basically forbidden in the openSUSE build service
because they cause issues but I don't know the details.