It will sometimes mess up the text sizes if you are changing screen sizes to a smaller size, but many times it resizes everything just fine. Going larger is pretty solid, although you still want to go look at all the pages and make adjustments as needed.
I can't remember if I did a conversion for a G3 that uses the buttons on the left and what the software does to those.
I've converted a G3 Kadet to CR1000 with no issues whatsoever. The screens were the same size, which helped. I didn't have to convert/adjust anything at all.
Like OkiePC said, you'll want to see if the bezel buttons are in use anywhere and come up with an alternative if they are.