Rule 355.11.b

If the group of targets no longer collectively fulfill the targeting restriction as the spell or ability resolves, that spell or ability’s controller can choose a subset of the original targets that fulfills the targeting requirement for the spell or ability to affect. Example: A player plays Fox-Fire, a spell that says in part “Kill any number of units at a battlefield with total Might 4 or less.” That player chooses four 1 [M] Recruit tokens at a single battlefield. As a Reaction, another player gives two of those Recruits +1 [M], so the Recruits’ Mights are 1, 1, 2, and 2. Then Fox-Fire resolves. The Recruits no longer have total Might 4 or less, so Fox-Fire’s controller must choose a legal subset of the original targets to affect. They could choose to kill the two 2 [M] Recruits, or the two 1 [M] Recruits plus one 2 [M] Recruit. The units they choose are Fox-Fire’s remaining legal targets. They can’t choose to affect units at the same battlefield that weren’t initially chosen as targets. They can, however, choose to affect units that were initially chosen as targets that left the chosen battlefield before Fox-Fire resolved as long as those units are all located at the same battlefield.