WordNet sense 1 (capable of replacing or changing places with something else; permitting mutual substitution without loss of function or suitability):

interchangeable, similar, standardised, standardized

WordNet sense 2 (capable of being replaced):

replaceable

WordNet sense 3 (capable of being exchanged for another or for something else that is equivalent):

commutable

WordNet sense 4 (capable of being exchanged for or replaced by something of equal value):

convertible