I do not know whether the preload can be changed but I suspect that if you can, it would just be a matter of using different sized balls to achieve higher or lower preload. One thing to try first is press down on the bearing block while sliding it. These rails, especially large ones like yours seem to need some load on them to get them to slide smoothly. Also make sure that the "rubber" seals on the blocks have some lube on them to as sometimes they can stick a bit.
What are you planning to build with those massive rails?