To answer your original question, NEVER! You do NOT change refrigerant like you change oil. You can do more harm than good doing this.

Secondly, you need to start by connecting a set of manifold gauges and get your pressure readings. If the clutch is not engaging, there can be many reasons for it, but the most common is a low charge which causes the low pressure switch to interrupt the clutch circuit. This is by design to protect the system.

If the static pressure is about 85psi on both sides without the clutch engaged, then the low pressure switch should be happy and you can then move on to check the circuit otherwise.

Good luck,