WordNet sense 1 (marked by erratic changeableness in affections or attachments):

volatile

WordNet sense 2 (likely to change frequently often without apparent or cogent reason; variable):

inconstant

WordNet sense 3 (liable to sudden unpredictable change):

erratic, mercurial, quicksilver

WordNet sense 4 (such that alteration is possible; having a marked tendency to change):

changeable, changeful