I'm really lucky and I've had the same group for 3+ years. We've gone 4e to PFRPG.
We were using the FR campaign setting and for the longest of time I wanted to put players into my own created world. The time is near.
I have a story arc in place now that is prepping this shift. It goes something like this:
1. Party has been assisting a small lumber town and were out on a routine quest for the town.
2. Party has killed cultists from both Vecna and Orcus (Orcus and Vecna cultists in this story are both using the party to do in the other cultists) over time.
3. Party arrives back in town to find it taken over by Vecna cultists that want revenge.
4. Fight ensues.
5. Orcus cultists arrive. They are very strong and want to do away with party & Vecna cultists.
6. Vecna cultists start fleeing by means of 'portals'. Somehow party will be coerced into fleeing into portal to 'live'.
7. I think at this point an Orcus cultist will cast a spell that impacts the portal while the party is fleeing and BAM... transported to new setting. Magic gone haywire :)
8. I need a plausible story for this next part. I want to strip the party of all/nearly all equipment. Basically arrive in their skivvies. I just need a logical reason why. I can't strip the Sorcerers spellbook because that would completely break his class. My other players can find new weapons.
My idea was to de-gear them and throw some lower level encounters at them until they gear up. They'll have to equip some tree branches and rocks until they get a game plan going.
I just need a story to explain why they were stripped of gear (minus the sorcerer spellbook)