There may be ways of rescaling inches to metric, other more subtle scale changes will require that you apply a scale on a component by component basis.
You can scale components using the Scale Body function by working in the context of an assembly, but you'll have to watch out for fasteners and other components that exist in multiple locations as they may wind up out of position.
If the assembly is mated or constrained and you intend to go through scaling the components one by one then I'd suggest suppressing all the mating first then apply the scale and try unsuppressing them afterward.
