If you change Ext number's on phones and another tech try's to work on the system and you haven't documented these Ext changes it will be very confusing to them. If the system dies and you need to reprogram it, it could be a nightmare to figure out what DN goes to what phone. Why not just move the crossconnects to the right DN.
This is a Signature and not part of the answer, it appears on every reply.
This is an Analogy so don't take it personally as some have.
Why change the engine if all you need is to change the spark plugs.