When you put the new freewheel on, be sure to coat the hub threads with a layer of grease to prevent seizing later on.
Best to put the tool in a vise lay wheel flat on it pushing down and slowly unwind. You don't want the wheel hopping off of the tool.
Probably best to let a bike shop do this.

New freewheel needs new chain.

Shift the gears before the load is applied.