Is the person allowed to change his card if the stack is not collected by the opposition?
Players must follow suit if they are able.
If unable, then before playing a card, the player must ask for trump to be revealed (or simply reveal trump if they are the declarer). If trump has already been revealed, then the player may play as normal (any suit).
From the rules at wikipedia:
In the case of a misplay, the hand ends and

the player forfeits the points he called in the start or the opposite player gains the points which they made at the start.

There is no provision for correcting/adjusting the misplay. The only remedy is the loss of the game points.