Hi,
I'm actually in the middle of migrating a client from 1.6 to 1.9.2.2 right now.
The process is rather straight forward. The biggest issues are going to be QA after the new version is up.
My process is as follows:
1) Set up your code locally so I can code on my machine
2) install a new install of CE 1.9.2.2
3) copy modules over (community and local)
4) copy theme over
5) start testing
For testing, I'd test the following:
1) account creation/login
2) add product to cart
3) change qty in cart/remove items from cart
4) checkout
These are obviously the core functions of the platform. If they are working, everything else should work too.
ALong the way I'll look for bugs and fix them if needed.
When all is finished I'll upload the code to your server and point apache to the new directory. That way, if there are problems, we can easily go back to the old code without breaking anything.
Let me know what you think, and we can get started!
Regards,
Matt