[Samsung Galaxy Note 9 Guides, News, & Discussion] tx_dbs_tx:
Dirty Flashing

Most of us are aware of this but I'll post it for others so they can be informed. You should always do a factory reset and/or wipe system, data, cache & dalvic-cache anytime you are flashing a new ROM build. In this case you are Odin flashing back to a specific official release then using the leaked update zip to update from Android 8.1 to Android 9 Pie. Update zips are specifically created to patch your current software and special scripts are performed during the flash process that are unique to that specific build.. Therefore they are not standalone ROM packages that can be flashed with Odin and they must be flashed on top of the current software. However, these update zips REQUIRE a specific base software version to already be running on your phone. Update zips are created for a specific software build/version and unless the update script detects the expected software version/bootloader/modem etc., it WILL NOT apply the update. This is why you must be on a specific software version according to the update zip that has been leaked. In this case it's the leaked AT&T builds, hence the reason you MUST first flash back to RJA or another specific build, then apply the update zip successfully. This is why specific builds must already be installed on your phone before you can apply the leaked/beta builds.

Dirty flashing is NEVER a good idea. In certain situations it's ok and in the case of update zips it's designed to patch your current software and wiping/factory reset isn't needed at all. But when flashing a full ROM from an official Odin package, it is highly recommended to delete all fingerprints, wipe data & cache then do a factory reset before Odin flashing the base ROM. This would especially apply when flashing custom ROMs with a custom recovery but that doesn't apply here since these new Samsung's have locked bootloaders. If you want to ensure zero issues from leftover data and data corruption or hardware issues like wifi & Bluetooth, you need to do a factory reset either from settings or the stock recovery BEFORE you Odin the base software and apply the update zip (Pie leak). Being lazy usually invites issues and nobody likes unnecessary issues from "dirty flashing". Please note that dirty flashing as it has been mentioned in this thread is perfectly fine as long as you did a factory reset prior to reverting back to the base software using Odin. It simply bypasses the first boot and setup process which saves several minutes. HOWEVER, I expect certain databases MUST have already been written prior to flashing an update zip. First boot is critical for the entire OS to operate smoothly and I highly recommend to allow the base ROM to fully boot and setup BEFORE applying any update zip. Take this advice if you don't want unnecessary issues..

Restoring apps and app data after upgrading the software can and usually does cause issues. It's now a feature in Samsung's software but to be certain, always clean flash and install all apps fresh from Playstore vs restoring from a previous backup. there's always a chance a backup can become corrupt or cause issues. In my opinion, it's simply not worth the risk and being a perfectionist myself, I always install everything fresh and never restore apps & data. I also love that clean, fresh feeling of a new ROM. I think we all do.