Inplace upgrade (like a normal upgrade but now with the same build). Mount ISO of the same build in explorer and run setup.exe.
Did you try to open any 1903 CU cab file with 7-zip and see the contents? or check C:\Windows\servicing\Packages after installing it you may notice there are no more numbered sub-packages for the CU (Package_1_for_KBxxxxx, Package_25_for_KBxxxxx,... files) instead you find new versions files for the default system packages 1903 CUs are truely service packs 3/4 part of the mess is caused by the above change, PSFX thing did not fit well with that change
Short answer: I haven't tried on 1903. I have known from a while, primarily from your posts, that the CUs are in fact Service Packs and a sort of rehearsal for the next release 19H2 delivered via Service Pack like mechanism. I admit that until now I didn't pay attention to the (dis)connection between the CUs being Service Pack like and the normal Windows update packages.
What about the RTM for release preview that coming today as well as rumored or is that not happening?
It's good if we get any 19H2 RTM/RC by today, by CU/DU, then the ESD/ISO would (probably) release by the end of this month, orelse earlier or mid Oct-2019. 1.40hrs to pass & see what MS has in their sleev for today.
Success. Those who've stuck in the earlier 19H2 18363.327 for the Resetbase/cleanup or whatever reason like me & didn't found any solution, with this latest CU, it's been updated to the latest version i.e. 18363.356 But, still, MS didn't fix this. UPDATE: Fixed Now.