Ok I think I found the answer,
See: Amendment No 2 to ICAO Doc 4444 15th Edition 2007.
15.2.2.3
b) having initiated the turn:
2) if able to maintain the assigned flight level, once the aircraft has deviated 19 km (10 NM) from the assigned track centreline, climb or descend to select a flight level which differs from those normally used by 150 m
(500 ft), if at or below FL 410, or by 300 m
(1 000 ft) if above FL 410;
It makes sense though considering that above FL 410 one is allowed to climb or descend 1000 feet. If at FL 430 one descends 1000 feet he will be at 420. So its better that the guy at 410 just climbs 500 feet. I dont know if this was the logic for the amendment but that's the way it is. The wordings have no ambiguity.
However getting access to ICAO docs is a nightmare. It took me almost 2 hours to find the relevant docs
