Tired of your old campaign? Are the PCs getting somewhat institutionalized, with no new challenged. Do you want to create a new campaign world, but still retain the old PCs. These are suggestions for switching campaigns gracefully, but also spectacularly. The idea is to give a reason and background for the shift, and make it something more than "Bob, the game master is tired of his old campaign, so he said that we found a cubic gate and want to go plane hopping."
In order to switch campaigns, "kill" the PCs. Actually put them in a no-win situation where they will all die. Remember, you want ALL the players DEAD! Leaving one or two alive can cause problems since he or she may want to resurrect the dead PCs. Also, do not arbitrarily kill them, make them go out with a BANG, not a whimper. It is kind of unheroic to be told "Well John, you were walking in a dark ally, and some assassin snuck up behind you and stuck a knife in you. You're dead."
[In my campaign, I had the PCs fight an entire wagon train of vampires and attendant werewolves. However, the PCs, being their normal perverse selves, almost destroyed the vampires. They tracked the wagon train back to the nasties' lair, which had an entire village of vampire and werewolves. Even high level characters will be overwhelmed by an entire village of vampires and werewolves, and the characters did destroy a lot of them in a knock-down drag out fight, and they went out with a BANG.]
The idea of course is not to destroy the PCs but to give an imaginative way to shift campaign worlds. So introduce several NPCs before they go to their doom, and have one of them be a disguised demigod on a recruiting mission. It would be great if the NPC is introduced early, (not as a demigod but as someone of moderate competence but not overshadowing the PC), the rationale being that he or she is scouting the PCs' abilities. When a PC dies, the NPC demigod collects their spirit/soul.
The follow-up campaign can start on a different plane or, as in my campaign, sometime in the future. It is up to them to learn about the new world that they are in, and the stranger the world from what they know, the more interesting it will be for them to explore. This is a good opportunity to "retune" the PCs, that is adapt their possessions to the new campaign by removing unwanted items.
[I started with the same PCs finding themselves naked on top of a butte. They saw two bodies near by, an unconscious man, and a summoning circle. They later found out that they have been summoned by the three into their future to either redeem or destroy their people. Turns out an evil force called the Blood God had convinced humans to wipe out most of the other races, and had then enslaved the humans, all but one nation of "good" humans. The old gods were no longer worshipped, and forgotten.]
If you shift them into a campaign setting into the future, you can tantalize the PCs with things familiar from their time. For instance:
[The PCs discovered that there was a "Tomb of Heroes" built just after their "death" and dedicated to them for their heroic services in the past campaign.]
History can also provide interesting side plots such as trying to get information about what happened to their people, or even how to regain what they have lost. For instance, to regain their favorite magic items, they might have to look in old dusty tomes to find it, and once they do, how do they get it back from whoever owns it now, if anyone. And their magic items may now have history, after all would it not be interesting to know that the that their +3 longsword named Brightwind was used in the Orc Campaign and is now known by the Orcs as Crouch Splitter. You can also have the magic items change, gaining/losing powers. Or else, as in my campaign:
[ The PCs went to the Tomb, where each found their best magic item from past with a special supplemental power thrown in by the demigods so that they could meet their god-ordained test. Along similar lines, they found that an old NPC they knew, a dragon, was now the grandaddy of all dragons and the only good dragon left alive.]
Prophecies and old histories work well. And what is written is never the same as what actually occurred. You could have fun with the effects of distortions on historical recollections. "Hmm, is that the church I established. Their interpretation of my writings was never what I intended, I guess I have to set them straight." Or perhaps their battles have been exaggerated out of proportion by minstrels. " Hey, he's singing about me when I held the pass at Dragon's Beak. Did I really hold it against 50 trolls by myself, I thought there were only 7, and I thought you were there Uruk."
If you are prophecy minded, the PCs might have to unravel prophecies to figure out why they are where they are. And if you do time translation, since it is into the future, you don't have to worry about paradoxes.
[ The PCs had to wade through old books of history they found in the Tomb of heroes and tomb of kings to find out that their reason for being there was to be judged by the gods. They found clues in old folk songs which spoke of them and their deeds, as well as what the gods had inspired prophetic bards to compose. They had to look at prophecies from the Red Book of Calcalsan the Mad, an ancient prophet who lived after the PCs time, and was known for his convoluted, but always correct prophecies.]
If you use this campaign shifting method, you need an epic villain, something other than that nest of werewolves to kill. A quest format may be appropriate where everything leads up to a final confrontation.
[In my campaign, the PCs were up against the Blood God who was an ancient elf who had disguised himself as a human to lead the humans to destruction. He wanted revenge because humans had destroyed his wife and nation in ages past. The PC's eventually got to the Blood God, after wading through giants, dragons, and his ogre legions, but was at a dilemma as to what to do with this elf, the last of his race. They chose to kill him instead of showing mercy. The major gods judged humans as flawed and rabid, with no capacity for mercy, and decided to destroy the world and remake it.]
This led to another, more spectacular way of changing campaigns.
[ In my campaign, several of the lesser gods, including the one who had chose them, sided with the humans and took the most worthy of them with them through a tunnel of light to another plane. So began a third campaign, the exploration of a new world, rebuilding of a civilisation, and the evolution of the PCs into demigod hood. ]