Change levels are indications of changes made to the engine while in production. As an example, change level 14 installed heavier valve springs. It was done in September 1966 for both Cleveland and Windsor built engines. Back then it made a difference in engine rebuilds. Today not so much as the variety of parts has been reduced so whoever might rebuilds your heads, may or may not use the heavier type valve springs as there may be only one type available.
Over the years, rebuilders tossed all the tags (engine, carburetor, rear axle, etc) as they didn't care what it was, they just did it.