So do I have to play all the missions again now that there is steam achievements

[Courtesy]

Or is there something I can do to not have to play all these again?

[Jacobkolstad]

You have to play the levels again to get Steam achievements.

[Courtesy]

That's a shame. Oh well... I'll just leave them like that then.

[Asbestos]

You only have to replay the missions with their own Steam achievements.

[mzimmer74]

Additionally, if you happen to have a save point prior to the mission ending it works fine to load it up and complete it.  You'll get the achievement.