It depends on the output pressure of the HPA tank. Some are "low pressure" with an output of @ 450 p.s.i. If it is "low pressure" output, you'll need to rework the springs and valve tube considerably.
If it is a "high output" regulator, @800 p.s.i you may just need to lighten the valve spring.
Stock Nellies were designed to operate on CO2, which is at 850 p.s.i at 70* F at sea level.
Fearlessly, the idiot faced the crowd, smiling...