Historical Rules · 30 September 2011
Rule 701.8g
Official source text from the 30 September 2011 Rules release
A spell or ability may instruct a player to exchange two numerical values. In such an exchange, each value becomes equal to the previous value of the other. If either of those values is a life total, the affected player gains or loses the amount of life necessary to equal the other value. Replacements effects may modify this gain or loss, and triggered abilities may trigger on them. This rule does not apply to spells and abilities that switch a creature's power and toughness.