did she realize
that she realized
has she realized
then she realized
only+状语（副词、介词短语、状语从句等等）置于句首时，主句要部分倒装，助动词提前。根据“met”可知本句为一般过去时。
句意：只有当他们毕业20年后，她才意识到他们的变化有多大。
