how to make Mithral Medium armor count as light

[deadkitten]

So say I want some Medium armor to count as light armor without changing the material its made from. How would I go about that if possible?

[Jackinthegreen]

Is there a specific mechanic you want light armor for?  That might change how you go about doing this.

The halfweight armor enhancement from Underdark would work to change any medium armor to light, but it's kinda costly.