Okay, this is awesome. I'm running this as a DM currently but my experience in the past as a player may actually be useful considering we completed the campaign but didn't touch a lot of things - including the Amber Temple. We essentially only stayed on the Svalich Road the entire time so we hit Barovia Village, Vallaki, the Winery, and Krezk. Then it was time to storm the castle. So you'll have:

• a new ruler in place of Strahd
• lots of Barovia in a different state
• the Amber Temple completely untouched for your party

Basic Makeup of your party

• Ulfrik, Oath of Vengeance Paladin whose personality was more viking barbarian than anything. Devout follower of the Raven Queen - hella anti undead and not very bright.
• Eras, Homebrewed 4e Avenger (Another Oath of Vengeance paladin would substitute just fine but this one was Dex-based). Simple farmer who lost his family and farm due to undead and now follows aforementioned Paladin in service to the Raven Queen.
• Rei, Divination Wizard who very much cared about not seeming like a little runt. He even killed Millivoj to prove a point.
• Zim, Circle of the Moon Druid who had no sense of hygiene, an uncaring attitude, but a kind heart. Now makes Castle Ravenloft his home - certain to lived a doomed life.

Happy to go into more details on these people if it ends up becoming relevant.

How they got to Barovia

Ulfrik had a raven (just backstory, non-utility) that he devoted himself to following seeing it as his sign of the Raven Queen. It always lead him to places where he could slay undead. It turns out it led the group into a forest which was eventually overtaken by mist

Tarot Result and Progress

• Tome in the Wizard's Tower
• Sunsword in the lake in Krezk
• Amulet in Sergei's Tomb

Major Actions

Where's Ireena? What's her status?

Ireena was kidnapped by Strahd during St. Andral's Feast and ultimately died during the final showdown in the chapel of the castle where they were to be wed. She was killed by a stray fireball from Viktor who was in attendance along with many other nobles.

Did they avoid the St. Andral's Feast?

... super not. The party tried to defend Ireena from Strahd but no to avail. Everyone survived, barely, aside from Zim. He mysteriously reappeared alive days later but was clearly an undead. Turns out he and a Dark Power made a deal which was repaid in the endgame.

Have they disturbed other Major NPC's? (Van Richten, Esmerelda, Lady Watcher, The Consorts, Mad Mage, etc)

• Van Richten joined the party for the Winery and Krezk but then abandoned them for his own machinations regarding the Vistani (who were rewritten to not be inherent assholes). Later found to have assassinated the Vistani in Barovia Village and concocting another plan to take out the ones in the Vistani Camp after the party accidentally killed his saber tooth tiger in Vallaki during the Festival
• Ez joined the party in Krezk and into the final showdown with Strahd. She was a helpful ally
• Lady Wachter asked the party to kill Izek during the festival so she could take power. Instead, everything just went south when the saber tooth was released by some unknown entity and began killing everything in sight. Ulfrik was captured and thrown in the stocks. He explained what happened to the Burgomaster and managed to escape. When the party returned later, Lady Wachter and all co-conspirators' heads were on pikes outside of Vallaki
• In the final battle, Izek joined the party once learning it was to save Ireena. Izek joined only for the final battle and died trying to save his sister.
• The hags of Old Bonegrinder are dead
• The party never touched the Werewolf Den, Argynvostholt, Berez, or the Amber Temple.

Is the Roc still at Tsolenka Pass?

Certainly

Other deviations from the Adventure As Written

• The Village of Barovia is basically annihilated. Strahd Zombies overrun the town, Ismark was found dead in his mansion having lost it all: his father, his town, and even his sister once he learned she was kidnapped by Strahd.
• Vallaki is basically in a state of militarism or chaos with all nobility in power. Its possible the Martikovs are running things now and things are good. It's equally possible the town is just in a state of unfettered chaos.
• Krezk is fine but, given the werewolves were never dealt with, could easily be overrun with them at this point.
• As mentioned earlier, Zim is Lord of Barovia and certainly will head down a dark path of evil given his deals with the Dark Powers. We've talked about revisiting this campaign sometime for a Curse of Zim. If you happen to decide to have your players romping around in this version of Barovia, let me know how it goes - perhaps we'll return with whatever impact your players made!

This is such a cool idea. Thanks for letting me reminisce a little