I'm sorry if this has been posted before, but it's been bothering me for a while now.

If a song's key changes from a Major Key, to it's Relative Minor Key (or vise-versa) Does it count as a key change? On one hand, it has a different tonic, but on the other it's impossible (or at least very difficult) to tell if you were looking at the written music.
Yes, it's considered modulation. Music works off the intervals between the notes, not the notes themselves. So even though they both contain the same notes, no Major scale is the same as its relative Minor.
When changing from a Major key to its relative minor key, as I've heard we say it's a key change though we are dealing with the same key.

But I'm not familiar with how to show that the key has changed. I'm also interested.
A key change is a key change, it doesn't matter what you've changed between. To switch betwwen the relative minor and major keys the arrangement of the chords has to change. It's not often a writer would choose that modulation as it's very subtle, the whole point of changing key is for it to be noticeable so a shift to the parallel minor would be far more likely.

Also I don't think you were alluding to this, but it's worth re-iterating that changing "scale shapes" does not count as changing key.
Actually called Mark!

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