i have some old bill cards from when it was a trainer card. if i play with modern rules, are they counted as trainers or supporters? and can i even use them at all?
They are to be played as Supporters (as per the most recent printing of the card) - regardless of whether you're playing Modified or Unlimited.
If you play old-school Bills, you have to play them as Supporters, and you must also have a current reference of the card to go with it (to be kept outside of your deck for viewing).
