Thanks for fast response. So does it mean that I would lose my achievement points from acquiring the old difficulty-related achievements? :/ It seems quite unfair and pointless. Also I hope that the rest of achievements will remain intact, right?
Umm... sorry for being so uninformed but... what are the feats of strength exactly?
Feats of Strength are achievements that are unfair or impossible to get, either because of luck, timing issues, or in this case because the part of the game involved no longer exists. They do not count toward your achievement points, so yes you will lose the points. However, the maximum points will also go down accordingly, and it's not like achievement points actually do anything for you.
As far as I'm aware, the difficulties are the only achievement-related feature that are being removed, so I don't think any other achievements will be going away.
OK, so I'm already playing "Starter Edition" only. Let's assume that I'll buy "Diablo III" and "Reaper of Souls" at the same time. Then upgrade the game to the full version. Then upgrade the game to "Reaper of Souls" level. OK, so how I'm supposed to get all the Nightmare, Hell and Inferno related achievements, if classic difficulties are gone?!
Achievements related specifically to the current difficulty modes will become feats of strength. They won't be in the standard list and won't count toward your achievement total anymore.