First to the OP ... I truly wish I had a campaign world I've worked on and build up as long as you have. Kudos to you! And as said before, I'm glad you're going to try 4e even with your reluctance. Btw, the 4e Island is a brilliant idea.
I'm in the middle of running my 3rd campaign in my homebrewed world. The 1st campaign I ran ended with a BBEG opening a portal to the Abyss to unleash his minions upon the world. Effectively making the Prime Material Plane the main front of the Blood War. I'm going to use this as a "bridge between two ages" in the
Zeitgiest Article. This will start an Age of Pestilence as corruption and darkness will spread like a disease across the world destroying kingdoms and countries alike. Over the course of 500 years, two great kingdoms will arise and fall ... the Tieflings and the Dragonborns. The Tieflings growing powerful because they were the only humans who made the pacts with the Infernals and accepting their role to help end The Blood War. The Dragonborns growing powerful overnight because of their bloodlines and ties to the dragons in the world.
I have a area called The Great Rift in my world where ages ago, the dwarves united the good races to fight back the evil races of the Underdark. It is in this "Great Rift" where a "last stand", if you will, will take place to end The Blood War. The fallout of it will be the dwarven race being enslaved by giants and the beginning of a new age.
During this Age of Pestilence my world will have it's "reset button" pushed, as it were, and from the disease of corruption, war, and darkness there will be small points of light popping up throughout the world. And I plan on using the town in the DMG, the one built on the ruined city, to start my 4e campaign. These new characters, adventuring on ruins the old characters helped build up, will find truth in the legends they grew up with and help start a new age of their own.
And that's how I'll be making the switch ... jumping in with both feet.

If I had as many years invested in my world as the OP has in his, I'd totally be stealing the 4e Island idea from him and not hitting a "reset button".
Thanks for reading this long post ... I kinda rambled on a bit.
Take care,
Dizlag