[sorrygottarunway]

Okay I just scratched a few things out on paper and this is what I came up. In the US, we reset our altimeters to 29.92 passing through 18,000, regardless of the actual altimeter setting we had at 17,999. Therefore, if it was actually 29.41, and we then dial in 29.92 in the kollsman window, it will show us actually climbing a fair amount without the aircraft physically moving UP in space. So we'll now descend back to where our altimeter reads 18,000 and be happy.

Now, take an aircraft that wants to cancel IFR from 18,000, and start a VFR cruise at 17,500. UH OH!!! He descends and sets his meter to 29.41, which on our altimeters lowers the indicated altitude. He then climbs to maintain that 17,500 for his cruise.

You can see where this is going- the two aircraft are no longer separated by 500ft, but rather have moved closer to each other, simply by doing the correct thing by setting the required kollsman values.

Sorry for the confusion, answered my own question!