noninterchangeable

from WordNet (r) 3.0 (2006)

noninterchangeable
    adj 1: such that the terms of an expression cannot be
           interchanged without changing the meaning; "the arguments
           of the symmetric relation, `is the father of', are
           noninterchangeable"

from The Collaborative International Dictionary of English v.0.48

noninterchangeable \noninterchangeable\ adj.
   1. (math, logic) not able to be interchanged without changing
      the truth value.
      [WordNet 1.5]

   2. Not able to be substituted for each other; -- of objects,
      such as parts of machinery; as, the ink cartidges on the
      new printer are noninterchangeable with those on the older
      model.
      [PJC]