Nah, you're overthinking it. You followed the instructions exactly -- you worked with one system of divination exclusively for at least a year, and then you went on to another. That doesn't mean you can't occasionally go back and use the first one! In the long run, you'll probably have a whole series of oracles you can use whenever you like -- that's where I am, for example.

If you feel unsure of your prowess with the SGO, you can certainly cast a couple of readings with it just to get back in the swing of things, but I think you'll find that you can pick it up and get good results with it right away.