You can change the number of sides on a die. If you don’t have any five-sided, seven-sided dice, or d21s handy, you can use other dice to simulate the rolls.

You can change power or toughness. If you change a toughness of 1 to 0, the creature will probably die.

Indefinite articles such as “a” in “draw a card” aren’t number words and can’t be modified.

You can change the number of dice rolled for an effect only if the original instruction included a number word to describe how many dice to roll. You can’t change the “a” in “roll a six-sided die,” but you could change the “two” in “roll two six-sided dice.”

You can’t change any instance of X.

You can turn a 0 into a -1. Sources that would deal 0 or less damage don’t deal any damage, even in silver-bordered games.

You can change the numeral inside of a mana symbol, but you can’t modify the cost of a spell in time to pay less for it. It must already be on the stack to be a valid target for More or Less.