Going by the REG, I'd say Unity has to be true when it's played, and after that it only stays active while Unity is true but would remain placed even if Unity is false.
REG:
"Unity is an identifier on some cards that appears in the format "Unity: X (Y)" and means "If
all of your X are Y". If a card has the unity identifier, the special ability and abilities in
identifiers (such as holds, activate, and taunt) can only activate if the unity condition is true.
Unity is not true if there is not at least one card that fits in X. If unity ceases to be true after
an ability is activated, no abilities on the card can be used until unity is true again."
