In an attempt to keep the "hassle factor" to a minimum I would advise going to your largest pot on your next transplant. This does a couple of things. First, it negates any chances of transplant shock. Secondly it gives the roots plenty of room to expand, both downward and outward without any chance of becoming rootbound. Plus, it gives you more room if you plan on training your plants to maximize yield. I'm going to be starting a new grow in a few weeks and I'm going straight to the largest pots my garden will accommodate.