Right- this. It's legal to grow with a license already (state wise anyway, not sure about federally but I digress)-- if you're in jail for growing, you did so without a license, illegal then and illegal after if the law passes. And-- same with what the guy said above- if you're in jail for weed, you did something that would still warrent jail. If you just got an MIP or something you wouldn't be in jail.
The real question would be if MIPs and stuff that would now become legal would stay on your record, and I believe the answer is yes because you were breaking the law at that time. There may be an asterisk next to it or something though.