Achievements have been changing over time because of balance updates. Although that makes some achievements very hard to get, that is not really a problem. What is a problem, is that there are several achievements which you simply can't get anymore because of changes to the commanders.
For example "Defending the Motherland" (Kill 100 infantry units with Soviet Irregulars) and "They're Coming Out of the Woodwork!" (Deploy 50 Partisan LMG Squads, 50 Partisan SMG Squads and 50 Partisan Rifle Squads) can't be achieved simply because the unit doesn't exist anymore. As a consequence, the Infantry bulletin medal also cannot be achieved.
Are there any plans on fixing this? I sincerely hope so.