You left out Karen Valentine. 
… And Monie Ellis in the '70s.
Yeah, but I’ll give them credit for Louise Tate.
And one of the reasons Alice Pearce was thin was because she was dying. Maybe the producers wanted a Gladys who looked healthier.
It’s been a while since I’ve seen them, but as I recall the main characters in the Atlas Shrugged trilogy changed in every movie.
An observation on this from Daylight Atheism:
Professor Bernard Quatermass has been played by several actors since his first appearance in the 1950s. Most of them don’t much resemble each other.
My biggest complaint isn’t that they’re all so different, but that the first two movie Quatermasses were played by Brian Donleavy, who doesn’t look or sound so much like a British Professor of Aeronautics as he does an American Gangster. Why not use the guy who first played him in the TV serial?
Plan Nine from Outer Space, Bela Lugosi died early in filming and was replaced by Ed Wood’s dentist, who was much taller.
The mother on Fresh Prince of Bel Air.
Maggie Gyllenhaal replaced Katie Holmes in Chris Nolan’s Batman movies. Both skinny brunettes but otherwise look nothing alike.
Maria Bello replaced Rachel Weisz in The Mummy movies. They gave Bello dark curly hair but they look nothing alike.
And of course Becky Connors on Roseanne, going from Lecy Goranson to Sarah Chalke and then back to Lecy Goranson. Both blonde girls, but otherwise they look nothing alike. And Chalke is clearly the better actress.
Thank you…it was so jarring i almost head-canoned them into two different people
Another wife-of in a horror film was Valerie Hobson in the 1935 Bride of Frankenstein, who replaced Mae Clarke as Elizabeth in the original Frankenstein, even though she’s a lot less blonde and looks significantly different.
Becky, as played by Sarah Chalke, once commented on the show that she thought the second Darren on Bewitched was better.
They also referred to it in the reboot: Sarah Chalke met Becky and they agreed on many issues, so much so that one of them said, “It’s like we’re the same person!”
The one that immediately came to my mind: The Partridge Family - Chris (the drummer) went from being a brunette (Jeremy Gelbwaks) to a blonde (Brian Forster) without explanation.
On Maude, Carol’s son went from someone taller than she was (in fact, taller than pretty much everyone else in the cast except Bea Arthur) to someone much shorter, although in this case, the fact that they look nothing alike has everything to do with the change, as they wanted someone whom people would believe was her son.
The two actresses that have voiced Meg on Family Guy - Lacey Chabert and Mila Kunis - sound nothing like each other.
The character played by Torri Higginson on Stargate Atlantis looks very different than whoever it was that played the same character on SG1.
Of course Richard Dean Anderson isn’t likely to be mistaken for Kurt Russell either for that matter, but going from movie to TV that sort of recasting is expected. Unless they suddenly change race or sex or something like that most people will be able to overlook it after a bit. But changing actresses from a show to a spinoff show set in the same “reality”, that’s a bit more jarring.
They intended to market the film in America, so chose an actorthat would appeal to that particular market.
The first TV Quatermass was Reginald Tate. The reason he didn’t return in the second TV series - he died suddenly shortly before filming.
That reminds me. Doctor Who has already been mentioned, tongue in cheek since the different incarnations are *intended *to look and act differently. However, there were two movies based on the series starring Peter Cushing. He looks and acts very differently to Hartnell, and this was before the concept of regeneration had been introduced to the series.
DJ Jazzy Jeff guested starred, telling the second Vivian Banks (played by Daphne Maxwell Reid): I don’t know what it is, but you’ve really changed since you had the baby.
Yes, that was particularly egregious. One could be forgiven for thinking he was a completely different character and saying “Hey, where did that pretty boy go? They made it out like he was going to be important!”
I preferred Chloë Annett(the toffy/nobby one) if only because I didn’t need closed captioning to understand her 
If you’ll recall, when Jack Lord first appeared on-screen, the audience (naturally) had no idea who he was supposed to be, and the movie let us at first think he might be a bad guy. Changing actors so frequently allowed them to use that same trick many times again in subsequent movies.
(Bernie Casey’s Leiter related a story about a pen Q had given him that blew up in his face. I always wondered if that was some sort of joke about why he was suddenly being played by a black dude!)
Indy
No show had more fun with this than Due South and the two Detective Raymond Vecchios. The main character comes back from vacation, and nobody but the main character notices that the guy who is supposedly his partner is somebody completely different.