The yellowing is perfectly normal. It's been explained to me that your plant is going to start using up the nitrogen in the leaves which causes the yellowing. Mine did the exact same thing and don't worry, it'll get even worse in the future but it's still totally normal. Post some pics so I can be certain. As for the switching of lights I'm not sure except by my own experience...I switched over to HPS right away. I've also heard that it's a good idea to give them, before the transition, 24 hours of darkness to speed up the process a bit. I didn't do this so I'm not sure if it works. There are lots of pros on this site, I'm sure someone will be able to point you in the right direction. Peace and happy growing!