I got curious and had to go do a little research on the 'net for this one! From what I can tell, the difference is that the BB 90 standard has bearings that are inside the BB shell. The BB 86 standard puts the bearings outside the shell. Otherwise, things are the same. So, it seems to me that you could put the crank and its axle from one bike into the other bike without changing anything else.

It is worth a try at least!