Star Trek - The Reboot
I know, it's not quite a reboot or a retcon. The time travel rationale (which has been used before in the Star Trek universe) that allows for this "alternate reality" Star Trek allows us to enjoy all the old Original Series episodes and Next Gen episodes along with whatever they're going to do with this rebooted franchise.
Still, this is done all the time in other media. When the venerable Gundam anime series had already established dozens of episodes in the original universe, all of a sudden: alternate Gundam universes starting popping up with similar characters and recognizable mecha designs -- the most successful of which was the Gundam Wing series. Interestingly enough, they still kept releasing shows set in the original universe -- sidestories involving new characters or adventures in the past or the future of the original series.
Comic books are notorious for retcons and reboots. Marvel has done their Ultimate version of their universe, where people swear and are nasty and betray one another and look a lot like well-known actors. Wait, that's the Ultimates. Ultimate Spider-man was actually pretty good as a reboot, getting back to the heart of the character and reinventing it for modern audiences. Before the Ultimate line, however, there was the short lived Heroes Reborn series of comics, which had mostly good art and hit-or-miss writing.
DC is the champion of the reboot, of course. They did the famous Crisis on Infinite Earths, allowing them to reboot everything -- condensing their multiverses into a single universe! And then just recently, they did Final Crisis, allowing them to bring back the multiverse (and all those alternate reality versions of their primary universe, of course).
There'll always be naysayers -- people unhappy with the latest incarnation of the characters and the storylines -- but if the stories are good, if the TV shows are good, if the movies are good, then the characters and the universes they inhabit should flourish as they attract more and more fans.