What exactly do you mean by upgrade ? Lighter materials ? The ash body on the JB-75 is pretty solid, neck dive is rarely an issue there. Higher gear ratio ? The figure of 20:1 sticks in my head so you could try these Aklot 23:1 tuners. AKLOT Electric Bass Guitar Tuning Pegs Machine Heads Tuner 23:1 Chrome Advanced | eBay Harley Bentons have smaller pegheads than Fenders so replacements like Wilkinson will be too big.