Mirror Universe

If an effect says that a player can’t lose life, that player can’t exchange life totals with a player who has a lower life total; in that case, the exchange won’t happen.

Wotc — January 01, 2011

When the life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player’s previous life total. For example, if player A has 5 life and player B has 3 life before the exchange, player A will lose 2 life and player B will gain 2 life. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them.

Wotc — September 16, 2007