No NPC names have been changed. I try to avoid that when at all possible. Chathi replaced the NPC she did because she is from official source (the Haunted Lands trilogy) and he was not. As for these you're asking about, all three still have the same names.
I do want to say that one of the things I really dislike is people OOCly discussing where special questgivers are, what they want, and what items they are looking for. This is one of the big reasons that questgivers no longer always give their quest to the first person who pops along at the level they require and why some move from location to location due to any powerful reward they provide and the habitual metagaming that goes on to find and get it. I guess it's human nature to seek the path of least resistance and use whatever means necessary to get that special item you crave. But that doesn't mean I have to like it or encourage it. I'll provide some tips, but I really don't want people to start saying "Oh - that guy is in Area X, he asks for item Y, and he gives you this really cool item Z."
So first off, I assume one you're you're talking about Marco Addams. He was a PC of Jechal's from long ago during our Beta testing period even before we officially had the vampire template implemented. As for where he is now; where would you look for an obsessed vampire hunter, who may or may not be a vampire himself?
The half-orc sounds like Ezra Stoneaxe, one of the founding PCs of the Veiled Alliance. If you can believe it, Jechal also played him as well (but all PC members of the Alliance were turned into NPCs). So where would be a good location for a guy fanatical about freeing slaves?
And the last guy. Ugh.
Culin Thark's scarcity is because pretty much everyone now knows what his 'reward' provides, even if only a few people truly found out ICly. And that's my biggest gripe with metagaming - it ruins the surprise of discovering something new and unexpected on your own. So because of what his reward can result in; as a matter of game balance he may or may not appear, in one location or another, on one day or the next (or the next), depending on the undead population in Thay. On more than one occasion I've deeply regretted implemented undead PC templates in Thay as I feel they can easily destroy game balance without checks on them. DMs have worked with or provided NPC limitation for undead PCs on occasion in campaigns/events, but that is not really sustainable long-term. So then it falls to the players to have PCs that actually say something like "Sooo...this guy is a vampire. He feeds on us - on living blood. He's not normal. We should probably kill him." Of course, that's never happened in my observations as known undead PCs here have, so far, instead been treated by other PCs just as any other. And, as is smart of them to do so, these undead PCs haven't really provoked living PCs with their undead abilities. So as a result, it falls to me trying to implement automated systems to somehow limit or try to provide a bit a balance to these powerful PC templates. This change to Culin Thark is just the latest in a long line of attempts to somehow put system-enforced limitations on undead PCs which otherwise would have little reason why even more people wouldn't try to create one to get all the stat benefits and powers they provide.
But if someone(s) were to start a fanatical undead-hunting guild of PCs (I'm sure many rival zulkirs and their lackeys of Szass Tam would be more than happy to support it and maybe give them some DM-driven attention), I will definitely work to make this last special questgiver almost as accessible as he was in the past.