Hey @KeithH - great question! I agree with everything @Maka says here.
To answer your big question directly:
Do experienced players readily move between positions once they learn a song once (albeit with either a key change or a harmonica change to maintain the original key)?
Typically, you would NOT change position. So if Sonny Boy Williamson played a song in 2nd position, 99% of the time a player who is covering that song will also play it in 2nd position.
Now, as @Maka pointed out, you might changed the KEY of harmonica because a singer needs to raise or lower the key for their vocal range, but you’d simply grab the appropriate harmonica and play the exact same draws and blows on the exact same holes as you originally did when you learned the song. (This is one of the great things about harmonica. It’s so easy to change key…just grab a different harmonica!)
Now OCCASIONALLY (1% of the time or less) someone my change the position, as I mentioned in my Heart of Gold post recently about Willie Nelson’s cover of it in which Mickey Raphael takes the basic idea of what Neil Young played in 1st position and plays it in 2nd position. But again this is the very rare exception to the rule.
Regarding other positions, I think you’re correct in saying that the minor positions of 3rd, 4th, and 5th have a “darker” sound. Bear in mind that probably 90%+ of popular harmonica repertoire is in 1st and 2nd positions.
I think it’s fun to explore other positions, but as a beginner if you’re wanting to get into darker minor tonalities, if it’s not a financial hardship, buying a Lee Oskar Natural Minor is a much easier way to get those sounds then playing in one of the aforementioned positions AND it affords you the ability to play minor CHORDS as well which is a lot of fun.
Does that answer your question?