noninterchangeable

from the GNU version of the Collaborative International Dictionary of English.

• adjective (math, logic) not able to be interchanged without changing the truth value.
• adjective Not able to be substituted for each other; -- of objects, such as parts of machinery

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective That cannot be interchanged with another

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective such that the terms of an expression cannot be interchanged without changing the meaning