why was the name changed from stellar Lumen to stellar?
Also what has changed between from the old stellar to the new stellar?
Your question as the same my concern. I still confused about some different names on some exchanges such as STR on poloniex and XLM on bittrex. Why different?
Stellar is the name of the company. Lumens are the spoken name of the coin itself. Most often you will hear them together, "Stellar Lumens" in conversation or simply Stellar since that's how people have been expressing it so far (thus your understandable confusion). As for exchanges, I'm not sure why Poloniex has the coin listed as STR. It's XLM. Perhaps someone more knowledgeable than me can explain it better to you. But that is the gist of it.
Stellar = The company
Lumens = The coin
Stellar Lumens = (try to look at it as a statement) "The Lumens of Stellar"
XLM = The standard exchange name.
STR = Poloniex doing their own thing?