When I started growing in grow bags 3 yrs. ago I also wondered about re-using the Pro Mix from them the next season. Upon reading Epic Tomatoes written by our friend Craig, he says not to do it in the interest of keeping soil born disease from transferring to the soil mix. Well my whole reason for having to use grow bags is exactly because of soil disease in my in-ground garden plot that I used for more than 25 yrs to grow tomatoes. So I decided to not risk shooting myself in the foot and take the advice of someone I respect. And I can say I no longer struggle with bacterial disease in my grow bags. I know Pro Mix is expensive but my growing success is worth it. But to each his own.
Dan