Maybe try adding an IF/THEN statement to Pro/PROGRAM.
Set it up to add a Y/N parameter asking if in inches (or mm). Then IF units == Y (inches) - use relations, ELSE (use alternative relations - if applicable), END IF.
Give it a try. Not knowing your specifics this should work.
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Is there any way to convert relations when you switch units in pro/E?
I have some relations in a feature that are based on inches. When I convert to mm, the relations do not get changed and things fall apart.
(WF 2.0, M090)