I found the ones that the Appelgren surname was first Appelquist very interesting and wondered how did that "just change"?
Appelgren is probably not a family name, but rather a one generation surname given to a soldier so that not everyone had the same Andersson, Johansson or Larsson. It would be a mess in the army if too many had the same name. And because of that the name could change a bit without no one cared too much about it. The name was often chosen from the farm he was representing as a soldier, in the example given by Roland earlier the farm is called "Apalle". And even if it was a one generation name, it sometimes remained, like in your case.