My Alistair remained a Warden and I thought he'd aged pretty well. I had no problem with him. I mean, they will always vary since the graphics are completely different. Characters that appeared in all three games have always differed greatly from game to game. (I was very unhappy with Alistair's DAII design. I don't think it's the devs' fault, though. The engine is what it is.) I find his DAI appearance, as a Warden in my case, perfectly fine given the new engine. I do feel like he generally became fairer in both skin and hair - he was rather tan in Origins and his hair was darker blonde, I think. However, overall I thought he looked pretty great and I could easily tell it was him.
On the other hand, I felt that Morrigan's features changed a lot (Her face shape, her nose, etc.) but... I love her DAI appearance. I think she's very beautiful, looks more real and she has an amazing smile. I got very disappointed, though, when she got rid of the gorgeous dress from the Winter Palace (It looked so stunning on her!) and dressed in the rags from Origins. (Why is she still wearing that all the time? It's been 10 years! If she really needs to show off her assets, she can do that while wearing something new.)
Now Cullen... Hell, Cullen was a shock. He changes a lot between all three games, but DAI turned him into this big piece of eye candy... and, I mean, I'm not complaining because... damn, look at him, DAMN. But I don't know how to feel about it because it's such a huge transformation and I can't tell whether it's because they decided to give him some love and thought it more fitting for his character (which I'm hoping for) or because they just tried to please us since he's romanceable now. I sure as hell wouldn't recognise him if the rest of the character wasn't there, though. Then again, given that he was supposed to be very young in Origins (The Toolset actually spoke of 24, if I'm not mistaken, which eventually changed to around 20 in the story.), I've never understood the design choices behind his Origins' appearance as he definitely looked too old for this age. Heh, Cullen is complicated. I like him in all three games, but I can't say I'm unhappy about what he looks like in DAI. 
Leliana's curious. I feel that her features are longer and less round in DAI. On the other hand, I quite enjoy her appearance in DAI. It has a bit more flavour. But I feel that her skin is almost unnaturally smooth, like it's not even real. I can't really complain about her, however. Really love the outfit she has going, too.
Cassandra is recognisable, but definitely different. Her nose and the shape of her face especially, I think. Again, I totally love her in DAI, though. I think they captured her personality so well and she's very beautiful. Happy to see the scars, too. They suit her. (Scars are so cool. I always want to ask about how a character got them. Same with Cullen.)
Varric looks totally different to me, though. I mean, I can easily tell it's him as he has many other trademark features like Bianca and all his chest hair, lol, and that's a good thing, but his facial features changed completely, in my opinion. He used to have this huge jaw going, bigger forehead and quite uniquely shaped but straight nose... I feel like his face became much rounder in DAI. But I have no issues with either version of him, I suppose.
I was glad to see Flemeth was more like an old woman again. I think the DAII version was a little too crazy and anime (That hair. Seriously.) and DAI didn't fix some of that for me (I don't think it's very fitting for the setting.), but I liked what they did with her face. I can see how much love went into it and I think it suits her perfectly.
As for Hawke, I couldn't get her quite right. But given that the graphics are so different, I didn't mind trying my hand at making a more realistic version of her. I quite enjoyed the result, even if it wasn't the same. (I don't think that's even possible with how different the games are.) What bothered me a little was that the short hairstyle wasn't available any more. I really liked that one on her in DAII.
And Teagan got old beyond recognition. Then again, it's been, what, 12 years. I wouldn't recognise him if I weren't told it was him, but I didn't have a problem with it. I absolutely didn't recognise seneschal Bran at all and it took some time before I realised who he was. The fact his hair colour changed didn't help. I probably wouldn't recognise Anora, either. None of them looked bad, but I couldn't tell it was them until announced or until they started talking (because I'd always recognise Anora's voice, at least).
All in all, I guess I can say I generally enjoyed what the old NPCs looked like in DAI. I felt like the new engine added new realism to them that wasn't quite possible before. With that came some new features, which is okay. I also remember it's been over 10 years since Origins. They definitely looked different, more or less, but I was always able to tell who was who, minus a rare exception or two. So, yeah, I suppose I like most of it.
Then again, that's just me, of course. 