Yes , the belts are easily changed if you need to go up or down a size , i just went from large to medium with mine . There were some good improvements from the 2014 version to the 2015 version , the floating lid (on the 3500) , slight changes in the belt , material for the lining in the bag , to name a few . I sold my 2014 model and got one of the 2015 versions . Fit and comfort IMO is about the same for the 2 versions . I don't think you would be sorry with either version . The Load Panel at $40 is a bargain and will do what its designed to do just fine
As to a RokSlide discount , I don't think so but Steve would be the guy at Exo to ask , I would think with all the RokSliders that bought one of these packs if there were a discount we would know about it , I know Steve's S & S Archery offers one , but Exo is a different business/website .