i find 'Direct3D 9 to Direct3D 10 Considerations' on msdn, but it's not practical

almost nothing is the same, and i have to rewrite everything from the beginning

The entire API was redesigned. You pretty much have to rewrite everything. If you are considering using D3D10 you are better off moving to D3D11 since it introduce the FeatureLevel system which allows you to support Direct3D9.

I agree that you should jump to D3D11, and skip 10. The APIs between the two are almost identical, but 11 gives you more flexibility.

However D3D11 support for D3D9 hardware is lacking a few rather important things that real D3D9 has, like Shader Model 3 and support for Windows XP.

Porting the code over isn't too hard, as long as you're using shaders and not the fixed function pipeline, but it will take a while. Unless you want to take advantage of the new hardware I'd stick with D3D9 for existing code, and consider starting with 11 on new projects.