Hi Ian,
In the past,I used to do Artexing.I have never artexed over artex, but I have plastered over many artexed ceilings. I suppose it would depend on the pattern that was already on. If the pattern was "rough",say stipple or swirl, you could probably put a "rough" pattern over it. But if you wanted to do a pattern with a comb, fan, half circle or any other pattern where you would need a smooth surface underneath, then I don't think you could. Be careful if you try to take old artex off in case it contains small amounts of asbestos. I didn't PVA any of the ceilings that I artexed, the vast majority were already emulsioned, and I just applied the mix to the ceiling, with no problems.
Roughcaster.