I have the original PRO. What is the difference between that and the PRO2?

Compared to the original PRO, the PRO2 boasts a few improvements. The PRO2 has an ergonomic silicone grip on the lever to provide a more comfortable brewing experience, and an updated portafilter that has 1) compatibility with our attachable single-spout and double-spout adaptor and 2) an improved hole pattern on the basket for better flow dynamics. 

Rest assured, if you have an original PRO, you can enjoy these upgrades (sans lever grip) by purchasing a PRO2 portafilter. It will be directly compatible with all of your current PRO parts (brew head, dispersion screen, tamper, etc).