It does seem that the upgrade to isometal alloy happens by the time of Prophecy however, given that average armor strength for light and medium fighters is about triple that of WC3-4. (e.g. 200 for the Piranha vs. 80 for the Arrow)We also know that the upgrade wasn't total, and it wasn't an immediate jump from durasteel to tungsten or isometal (indeed, I suppose now that isometal probably never filtered across to the frontlines at all - not if Righteous Fire took place after the war...). We see ships first being upgraded to plasteel, and only several months later do we see the ships of WC3.