The first thing that comes to mind is changing any Canvas objects to a FullCanvas if you wan't to use the full-screen capabilities. That means that if you have any Commands on those canvases then you'll have to get rid of them, and implement you're own "Commands" by drawing them yourself, and firing them from the keyPressed method (using the FullCanvas.KEY_SOFTKEYx constants). Then test you're program and if it runs well, you're done. If it's slow or ugly or something then you might want to look into using the Nokia specific classes to fix that.
Of course if you don't care about using the full screen then you really don't have to do anything.
Oh yeah, one more thing. If you want to use sound and the device doesn't support the MMAPI then you can implement that using the proprietary Nokia Sound class.