To bring the airport into line with ICAO regulations.
Also too many foreign pilots who did not fly in regularly were getting confused with the Alpha Numeric system, now they know that the first number is the terminal, the 2nd and 3rd numbers are the gate.
At the moment the cul-de-sacs are still referred to by the letters even though the stand numbering has changed, but from next week they too will change and become known by numbers. That will be confusing - you could have an a/c going to cul-de-sac 4 for stand 325 for example !!.
The biggest change will be the re-naming of the inner and outer taxiways and ceasing to use the "block" numbering system that is currently in use. From what I have seen there doesn't seem to be any way of referring to the exact part of the taxiway you are on with the new system, where as with the current "block" system the tower knows exactly where each a/c is and can get them to stop pretty much anywhere just by telling the crew to stop "in block number XX" or "turn left/right on the inner/outer at block number XX" to get the a/c to turn in the right place.
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"