F.R.A International Build Rules & Guidelines 2006

Para 2.4
If interchangeable panels and/or weapons are used, the weight is measured with the heaviest set-up in place.

However......
Para 1.2
If you have a robot or weapon design that does not fit within the categories set forth in these rules or is in someway ambiguous or borderline, please contact the event organiser. Safe innovation is always encouraged, but surprising the event staff with your brilliant exploitation of a loophole may cause your Robot to be disqualified before it ever competes.

Drive systems are not classified as weapons so really only Para 2.4 would apply in your case.

(Message edited by anukhet on August 19, 2006)