It wasn’t really memorable but Ed O’Neill did play Joe Friday in an updated version of “Dragnet” after “Married…with Children”. The series is only noteworthy today because Eva Langoria (“Desperate Housewives”) plays a detective several times in the second season.
Ricardo Montalban - from machiavellian Khan Noonian Singh in *Star Trek *to mysterious, supernatural Mr. Roarke in *Fantasy Island *to Senor Senior Senior in Kim Possible. 
Also, Leonard Nimoy went from logical Spock in *Star Trek *to master of disguise Paris in Mission Impossible.
Maybe you should present us with a list of shows you found memorable, so that we may properly satisfy your criteria?
Gotta love that smile! But we can’t bring up CKR without mentioning his Due South partner, Paul Gross. Paul went from upstanding Mountie in “Due South” to actually crazy theatre director in “Slings and Arrows” and stayed utterly believable. The only thing that Constable Benton Fraser and Geoffrey Tennant had in common (besides being Canadian) was a bad habit of seeing ghosts, and even that was pulled off in different ways. Paul’s current role as Darryl Van Horne is pretty distinct as well, although he occasionally reminds me of the Geoffrey.
I don’t want to rain on anyone’s parade but surely the point about Helen Mirren and Ian McShane is that they are NOT “TV actors” ? They’re classically trained.
The parade’s doing just fine.
By “TV actors” I think the OP meant “Actors on TV”–in order to exclude movie roles.
Besides, quite a few actors working on TV have had formal training. Even if that training wasn’t in Olde England.
There’s Howard Hessman as Dr. Johnny Fever and the teacher from Head of the Class.
Didn’t mean to come across as so crotchety. My point was simply that a lot of actors have multiple roles, even meaty ones. It’s the rare actor who meets the OP’s criteria – very different, memorable and within a few years.
Dr. Johnny Fever _ memorable. Guy from “Head of the Class” _ not so much.
Hugh Laurie is a great example of someone fulfilling all the criteria. His earlier characters may be a bit on the cult side and unfamiliar to most viewers, but they were still memorable. And there’s no denying Gregory House is also memorable and markedly different.
Neil Patrick Harris (Doogie Howser and Barney Stinson) would be a good example, but there was an awfully long gap between the roles. Twelve years, I think.
So forgive my attitude. I was just saying people should ask questions about HOW their suggestions meet the OP’s criteria.
I think what you meant to say was “His earlier characters may be a bit on the cult side and unfamiliar to most American viewers…”
Hugh Laurie is extremely well known in the UK, Australia, and NZ (and, one presumes, Canada) for his earlier comedic work, not only Blackadder, but also Jeeves & Wooster and A Little Bit Of Fry & Laurie. Lots of people- myself included- don’t primarily think of him as Dr. House (although I agree he’s an outstanding actor in the role and deserves all the credit and acclaim he gets for it.)
Not to mention Mitzi in Priscilla[URL=“The Adventures of Priscilla, Queen of the Desert (1994) - IMDb”]. Everything I’ve seen him in since then, I’ve thought “That can’t be Mitzi!”
Thomas Gibson. Greg Montgomery, a comic role, in “Dharma and Greg” to Aaron ‘Hotch’ Hotchner, the deadpan profiler, on Criminal Minds.
Edited to add: And how about Mandy Patinkin, who has among other things been a doctor, a grim reaper, and a profiler as well.
Shortly before he was Greg he was the closeted bisexual husband on Tales of the City, also a very different role. (Shortly before that he lived in my back yard pretty much- he was a member of the Alabama Shakespeare Festival repertory company.)