I Have Dawnguard, Hearthfire and Dragonborn installed. Use NMM, BOSS and TES5Edit (also run Mod Configurator, ATTK Skyrim Power Launcher, Skyrim Project Optimization, SkyUI/MCM, FX Post Process Injector, all the Unofficial Patches, about 120 plug-ins and about 80 replacers). Have a very stable game, virtually never CTD. See my sig for my rig's specs if needed.
~ Preliminary (Still @ v1.8) ~
1. Set Skyrim to not Auto Update in Steam weeks ago.
2. The day 1.9 went live, I launched Steam and went into "Offline" mode.
3. Launched the game w/ SKSE Launcher to be able to play while averting a forced update by Steam.
4. Let a couple days go by to allow ample time for SKSE, TES5Edit, Unofficial Patches and other mods (where needed) to be updated to be v1.9 compatible.
5. Created one last save prior to updating in an "owned" interior cell (in this case Proudspire Manor) and quit the game.
~D-DAY!!! (Update to v1.9) ~
1. Ran "Disc Cleanup" and defragged the main OS drive.
2. Went into Steam, set to "Online" mode and (temporarily) set Skyrim to Auto Update. v1.9 Update automatically downloaded.
3. In Steam, "Verified Integrity of Game Cache" for Skyrim.
4. Opened the Game's Launcher to allow: "Skyrim will now detect your video hardware and set options accordingly". Verified it all looked good. Closed Launcher.
4. Downloaded/installed latest version of SKSE.
5. Downloaded/installed latest version of TES5Edit.
6. In NMM, examined all the mods where a later version was indicated/available to be downloaded, to determine if any required v1.9. Replaced/Activated two that did.
7. Installed/activated latest version of all the Unofficial Patches (which require v1.9).
8. "Cleaned" all the Official Master .esms (Update.esm and the three DLCs) with TES5Edit as instructed in Gopher's tutorial video.
9. Ran BOSS and re-sorted load order to be identical to the load order prior to updating.
10. Generated a Merged Patch in TES5Edit and activated it in NMM.
11. Launched the game with the "ATTK Skyrim Power Launcher", went into the console and verified SKSE was present and @ the latest version.
Test played for about 15 minutes last night, walking, carriage and fast traveling, covering as much of the map as possible... without a hitch. Everything seems good

So Guys, I'm always hoping to further learn.... so how'd I do? Would I have been better off doing any of those steps in a different order? Did I leave any step(s) out? Thanks for taking the time to read (if you did) and thanks in advance for any helpful advice/critiquing!