When you play a new Play Rule, if the new Play Rule is more than the old Play rule. Take the difference between the 2 rules, and you can play that many more cards.
For example: If you replace a Play 2 with a new Play 4, you would get to play 2 more cards on you turn. So in total, you could play 4 cards on your turn (playing the Play 4 does count towards the number of cards played that turn).
If the new Play Rule is less than the old Play rule. You have to play less cards on your turn.
For example: If you replace a Play 4 with a new Play 2, you could only play 1 more card. As you now can only play 2 cards on your turn, instead of 4.
However, you never undo anything, so if you already played 3 cards, then replaced the Play 4 with a Play 2. Your turn just ends, you don't have to undo your other cards played.