Side Missions

[B.A.]

If I complete the campaign in recruit difficulty without touching the optional side missions, can I replay those side missions and finish what I didn't finish if I restart the campaign on new difficulty, say, normal mode?

[SpunkyJones]

Sure, why not? It would be a new game, unless I'm miss reading you.

[Nero]

No problem.
After finishing the campaign you can go to Mission Selection menu and select where and with which difficulty you want to get back in. If you select the right segment you can get to that Map again where you select the side mission that you want to play.

The difficulty does not matter, you can finish the game on recruit and jump in aywhere with any other difficulty. You can even lower it but not raise it while inside a mission by pressing escape.