There have been numerous examples of actors who previously played heroes appearing on superhero or sci-fi tv shows and films as other, unrelated, characters.
(This doesn't count actors meeting their later counterparts like Buster Crabbe on Buck Rogers in the 25th Century, Christopher Reeve and Helen Slater on Smallville, or Adam West on Batman the Animated Series!)
The Lone Ranger meets Commando Cody!
Clayton Moore (The Lone Ranger on The Lone Ranger tv series) played the lead villainous henchman in Radar Men from the Moon!
Commando Cody meets Robby the Robot!
George Wallace (Commando Cody in Radar Men from the Moon) was the Bosun in Forbidden Planet! (Bonus: Richard Anderson [Oscar Goldman on Six Million Dollar Man and Bionic Woman] was the C57-D's Chief Engineer!)
Spock meets RocketMan / Commando Cody! (sorta)
Leonard Nimoy (Spock in numerous Star Trek incarnations) as Narab, a Martian invader in Zombies of the Stratosphere!
The reason I say "sorta" is that the hero, while named "Larry Martin", wears the RocketMan / Commando Cody outfit, and the original script and promo materials list the hero as "Commando Cody", and the supporting characters, carried over from Radar Men from the Moon, are the same!
PLUS: the actor, Judd Holdren, then played Commando Cody in a series of new 25-minute films released to both tv and theatres entitled Commando Cody: Sky Marshal of the Universe!
Commando Cody meets Rocky Jones: Space Ranger!
Richard Crane (Rocky Jones in Rocky Jones: Space Ranger) played the sidekick in the Commando Cody: Sky Marshal of the Universe shorts!
Batman meets Superman!
Robert Lowry (Batman in the Batman & Robin serial) appearing as a government agent on Adventures of Superman!
The Shadow meets The Green Hornet!
Victor Jory (The Shadow in The Shadow serial ) playing a villain on The Green Hornet !
Dracula meets The Green Hornet!
John Carradine (Dracula in several films from various studios) playing a demented villian on The Green Hornet !
Captain Pike meets The Green Hornet!
Jeffrey Hunter (Captain Pike the first Star Trek pilot, The Cage) as a crooked construction business owner on The Green Hornet !
Captain Midnight meets Captain Kirk!
Richard Webb (Captain Midnight on the Captain Midnight tv series) as a psycho Starfleet officer on Star Trek!
The Rocketeer meets Captain Picard!
Bill Campbell (the Rocketeer in The Rocketeer) as a lovable rogue on Star Trek: the Next Generation.
Doc Savage / Tarzan meets Superboy!
Ron Ely (Doc Savage in Doc Savage: the Man of Bronze and Tarzan on Tarzan [1966-69]) portraying the retired Golden Age Superman on Adventures of Superboy!
James Bond meets Superboy!
George Lazenby (James Bond in On Her Majesty's Secret Service) as Jor-El in Adventures of Superboy.
James Bond meets John Steed (Twice)!
Patrick Macnee (John Steed in The Avengers and New Avengers) played Sir Godfrey Tibbett, an expert on horse breeding consulted by MI-6 killed by the baddies while assisting Bond (Roger Moore).
Macnee also appeared (voice only) in The Avengers feature film with ex-James Bond Sean Connery as the villain. They had no scenes together.
Macnee also re-teamed with Moore in the World War II film The Sea Wolves.
John Steed meets Sherlock Holmes (Twice)!
Patrick Macnee also played Dr. Watson to two different Sherlock Holmes who had played other genre characters!
Roger Moore (James Bond / The Saint) in Sherlock Holmes in New York
Christopher Lee (Dracula in numerous Hammer films, Count Dooku in the Star Wars series, and Saruman in the Lord of the Rings trilogy) in Sherlock Holmes & the Leading Lady and Incident at Victoria Falls!
(Lee had also played the villain Scaramanga opposite Roger Moore as James Bond in Man with the Golden Gun!)
John Steed and James Bond meet the Man from U.N.C.L.E.!
In the Return of the Man from U.N.C.L.E.: the Fifteen Years Later Affair tv-movie Patrick Macnee played Sir John Raleigh, the new head of U.N.C.L.E.!
George Lazenby (James Bond in On Her Majesty's Secret Service) cameoed in the tv-film as "JB", driving an Aston-Martin DB-5 and saving Napoleon Solo with the car's weaponry!
John Steed and Napoleon Solo meet Kwai Chang Caine!
Both Robert Vaughn (Napoleon Solo on The Man from U.N.C.L.E.) and Patrick Macnee (John Steed) appeared several times on Kung Fu: the Legend Continues.
Their characters are implied to be Solo and Steed whose identities have been changed when they retired.
Barnabas Collins meets Spock!
Ben Cross (Barnabas Collins in the 1990s revival of Dark Shadows) played Sarek in Star Trek !
While most of his scenes are with Zachary Quinto as Spock, there's also a deleted scene on the DVD of Cross and Nimoy (as Spock)!
There have even been meetings of two actors who played heroes, and neither was a heroic character!
Adam West (Batman on Batman ) met Alec Baldwin (The Shadow in The Shadow ) on 30 Rock!
Know any others?