I just received the 900 today, and I am looking for a lower shaft for the same reason, quick coil change, I did that for the Safari, the Legend and the EQ 800. Found nothing yet on the Inter Net or any Metal Detector sales outfit. So far anyway...
I was looking at pictures of the GPX 6000 and I noticed that the lower shaft looks a lot like the 900's lower. It has the red limit line and the same size and shape bracket where the coil connects. I'm going to call a dealer to see if they might have any info on it. It was priced at $65.