I have bought a 20 year old house and I'm not sure whether to rewire the whole house. I would like to change and add lighting and electrical points as well as cat 6 cables. If I rewire the house, do I need to change all drop ceilings and replaster all the walls? 

Also, the existing db box is 63amps 3 phase. Is this sufficient for a 2 1/2 storey house with 6 bedrooms?

I would rewire the hse for the following reasons.

20yrs ago switches are shoulder lvl, now is waist lvl. Lighting point location also vastly different from the past. To get the latest look, might as well redo the wiring. 

If old wiring start to fail, opening up false ceiling each time will cost a bomb. For commercial property.. Exposed Trunking ugly is ok, but resi not tolerated. 

63A 3 phase is generous. Just make sure to balance out the aircons over the 3 phase and not put all heavy appliances on just 1 phase. 

I agree with yoongf.  If you're already contemplating adding points, cat 6 cables, etc etc, might as well rewire the whole house.  Mine is a 16yo house, I also rewired the whole house.  Don't wanna deal with the hassle when any points fail, etc.