mainegrown has it right. Don't worry about transplanting to a different soil, it's gotta be done eventually.

You can see if they are rootbound by taking them out of the cup, or checking to see if the roots are growing out of the drainage holes on the bottom of your container, there's good instructions here: http://boards.cannabis.com/basic-gro...ency-baby.html

I personally transplant 4 times, makes for a nicer root mass when you let the roots bunch up a few times. I make the last transplant a week or two into flowering. It also helps to keep the soil's ph levels stable, as they tend to drop after a while in peat based soils such as MG.