If it's how I think it is then every weapon has it's own file that contains it's values such as fire rate, damage, range etc so technically they could change the values easily but it still has to be tested, this could be repetitive work and is probably why it took them a while to come out with a weapon balancing patch. The exotics though would be a little more complicated because of their special perks which are probably in a different file and referenced in the exotics weapon file so changing the Thorn's perk to stack and lower the damage ticks could also take a good amount of time