Some interchangeable items may be context-specific; suitable in one situation but not another. Screws of the same size are interchangeable, but not necessarily in precision machinery.
Pro Tip 2/3
Non-universal
Not everything similar is interchangeable. Some items or concepts may look similar but have unique characteristics. Nickels and quarters are similar coins, but not interchangeable when paying for an item.
Pro Tip 3/3
Beyond Physical
Interchangeable also applies to ideas, like words or roles, that can be switched without changing the overall meaning. The words 'happy' and 'joyful' are often interchangeable in a sentence.