There is also a conversion guide on Paizo's site that tells you how to convert from 3.5 to PRPG. It is mainly in the change in skills and feats that you would have to be careful with the old material. For example, requirements for prestige classes may have changed some because of the skill and feat changes, so some conversion will be needed. That and needing to calculate the new grapple numbers for monsters and npcs in 3.5 adventures.