Provide a smooth turnover
The most important thing is to provide a smooth turnover regarding the background of the campaign, the setting, your notes on what has already happened, and your plans for the future. Handing over a stack of papers is less than ideal, you should actually go over it with the new GM. In your particular situation, it sounds like you have already done this, which is good.
This won't apply in your situation, but if handing over a campaign to a truly new GM, it is worth taking some time to go over general strategies for running a game and provide advice on common difficulties. Of course, doing that if the person has experience as your GM does could be seen as condescending, but it is helpful for someone that has never had the role before.
It also sounds like in your case the GM will already know the players, but in general providing thorough introductions could be useful.
If you remain in the game, try not to be overly possessive of the world and be careful of unsolicited advice.
The hard part can come after the handover. It is very easy, and even somewhat justified, to feel an ownership over the campaign and world you have spent a lot of time developing. But if you want things to go smoothly, you should resist the temptation to be a backseat driver. The new GM will do things differently than you did and make different decisions. Bringing this up will likely cause resentment.
Remember, even if the new GM is building on the foundation you created, they are the GM now and falling into the temptation of holding onto the world you created before too tightly will likely help no one. In particular, I would try to avoid being offended if the new GM completely ignores the outline of future plans you have. They likely have or will soon have their own plans that are very different from yours, and that is not only justifiable for them it could be good for you since you generally don't want to know exactly what will happen.
The same goes for unsolicited general advice. For a brand new GM, providing advice is likely to be helpful, but providing advice like that to an experienced GM that didn't ask may be condescending. Of course, if the new GM does ask you should offer your full expertise, but that is a different situation.