If you ( or your builder ) have used trusses for the roof then the frame company designs the roof for the load. A trussed roof is slightly beefier to take the heavier load from a tiled roof. If you changed it later to a Colorbond roof all you would have to do is change the battens and lay the new sheeting. Going the other way I'd be asking the truss company or an engineer for advice.
Stewie