I've found that this action is rarely taken because it's quite uncommon to actually get the card you need to increase or decrease a challenge in front of you, so I'm thinking of adding the following to this rule:
You can substitute a challenge in front of you with any card that shares at least one suit with the challenge card, but you need to play as many cards from your hand as the absolute difference between the values of the two cards. So if the difference is bigger than 1, you'll need to discard cards from your hand until you have played the needed number.
For example, if you have a 9 challenge and want to substitute it with a 7 one, you play the 7 and discard one card from your hand.
I think this will make this action actually useful… Thoughts?