I've not done this (I had done so in an earlier version but took it out intentionally). The reason is I believe if something has been assigned 'no-unit' it indicates that the quantity does not and should not have units, for example strings. If someone attempts to convert such a quantity I want Iris to raise an exception and tell me this is not possible. This is what will happen as it stands. If I add ```if not self.units.no_unit``` then it will allow the user to convert from no_unit to any unit you like (making no change to the points or values). I feel that such an operation should be done by assigning to the ```units``` attribute, not by ```calling convert_units()```.