Take this FWIW...
I received the Pie update yesterday morning and attempted to install it around noon when prompted. I then got caught in a re-boot loop. This was w/ a US Unlocked S9+ on Sprint.
Luckily, I have a Samsung store nearby, so I ran over there. They were able to get be back to my original OS. They told me it was 'updated' (obviously not), but before I left the rep's desk I was prompted to d/l and upgrade again. I did w/ the rep there and I was put back into the loop again.
While there, another user was there w/ the same problem (T-Mobile carrier).
They ended up flashing both of our phones manually, wiping all local data, which was fine (all my data was sync'd), though now I seem to be missing some of my previously installed apps from the Google Play Store under 'My Apps', so it's going to take me weeks to figure out what I'm missing.
Beware.