What would happen if Jupiter gravitational pull was increased or reduced?

Before I answer your question let me quickly say something. The force of gravity is dependent on two things: the masses of the two objects in question and the distance between them.

Jupiter will not increase its gravitational pull unless it swallows a black hole or merges with Saturn.

So...with that being said, should the gravitational pull hypothetically increase, the orbits of the moons would decrease in a radius squared proportionally and increase in angular velocity.

Less stable  hope this