A record would have the default denomination associated with the system locale (in this case the system locale was set as Australia, so records with a value in currency showed AUD), but if a user opened the record and then switched to a different currency type, then the value auto adjusts (due to the currency conversion) and on return to the list, the currency field then presented a little globe/earth icon and you could then switch currency types from the list by clicking the icon (the range of currency types are defined in another system property).
I get the feeling that there is potential for things to get tangled up a bit, or for user to be confused, so perhaps that is why it is locked down out of box.
However, if you still want to go ahead and make it editable, then maybe one of the solutions is to remove the currency type of field from the array of un-editable elements from the System Properties > UI Properties module
