I'm going to lock this as it is double posted in towing section.
YES it is NOT illegal to up the gvwr of a vehicle. Aftermarket places do it on a daily basis. They can change the gvwr of said rig. A lot of things get done aftermarket to vehicles. Probably half of the motorhomes on the road have a different gver on them as they have aftermarket tag axles, or the body manufacture wants it to have a gvwr equal to the sum of the axles!
With this all said. The OP may be able to add the add a leafs and be fine. Then again, as noted, something else may break, warranty is out the door.....
Marty
92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
2014 Chevy 1500 Dual cab 4x4
92 Red-e-haul 12K equipment trailer