The characters are based in Fallcrest, straight of the the DMG. Reading through Fallcrest you can see that the trading houses have some amount of influence. This is continued in H2 as the Azaer's and Halfmoon's have outposts in the Seven Pillared underground hall.
So I used the hook that the trading houses and Lord of Fallcrest are concerned that reports from Winterhaven are not good and trade is being disrupted. It ended up that I had at least one character associated in some way with each major house's (Azaer's, Halfmoon, Naerumar, Lord Mayor, etc.) Each of them is asked to contribute one or more folk to go on a mission to check out the trouble to the west.
This will tie in well when we get to H2 because I can use the same association with the trade houses there.
H1 Changes:
Kalarel worships Vecna, but only his closest allies realize this. The portal can be to the Shadowfell, but the Far Realms works just as well (or better). I'm playing up a Mythos "feel" by references to ancient gods with no names "older than the gods you know, even", shadowy figures, "secrets not meant to be known". Everyone has some backstory or hidden secret that Vecna would love to know and leverage.
I changed the Kobolds and Goblins to worship Tiamet. They're being used by Kalarel. You can also tie this Tiamet connection back to Armos Kamroth in Fallcrest.
Winterhaven is a dour, sullen place. No one wants to talk. Think Lovecraft. People are having very disturbing dreams (lots of Far Realms/Mythos references - shadowy figures, people's faces oddly disfigured, strange tentacled gods). Everyone is on edge, but they don't know why. Caravans have been attacked, with all the people mysteriously gone. Farms have been abandoned (kidnapped) but assumed that they just moved away, etc.
I changed Douven Staul to be local to Winterhaven, with a wife and child in town in one of the tenements. Backhistory is that Kalarel and Agrid came to town, hired Douven (guide) to help with an archelogical expedition. Agrid, Douven, and the halfling occasionally come to town for supplies. Ninaran is in town and basically plays as written.
Secretly, Douven's son has been stricken with a strange disease that causes disfigurement. I.E. he's turning into a monster. His wife keeps him hidden away in the tenement and won't easily speak to strangers. Agrid uses this as leverage to continue to keep Douven in line. I used freeing Douven and curing the son as a Quest hook.
I changed the Dragon Burial site to instead be Keegan's Tomb. I moved rooms 5,6,7,8 out of the keep and to this location (thanks Sebastian!) Keegan was killed by his men and taken to this site away from the keep for burial. Agrid is seeking Keegan's weapon (Aecris) because of it's connection to the death's at the keep. His tomb is secret and guarded because of Aecris. It's now the McGuffin. Also, change Aecris to be whatever type of weapon that's usable by your players. In mine it's a Greataxe, for example. I then added a minor Quest for the players to redeem Aecris and recover Keegan's family heirlooms (room 15) back to this tomb to achieve his final rest.
The keep plays about as written. I cut rooms 5/6/7/8. I cut rooms 9/10/11 (booooring) and moved the stairs down to there.
The lower level I'm keeping basically the same. The Bloodreavers are involved because they've been trading slaves back and forth with Kalarel. Kalarel wants weak mind/bodies that are easily sacrificed and the Bloodreavers want strong backs. So Kalarel gets rid of troublesome folk and the Bloodreavers get rid of worthless stock.
The end game the players will discover the connection to Vecna and I'll include a hook/clue back to H2 since there is a Vecna tie-in there as well. Overall I hope to have Vecna folk behind the scenes pushing the buttons in many areas, even back at Fallcrest. Very old school.