Can I change the units on my team?

Yes, to do this, when in the mission screen, prior to accepting the mission, select the unit you would like to change. Towards to bottom left of the unit screen that comes up you will see a picture of a unit with a red cross, select that.

You will be back at the mission screen and prompted to ‘Add New Unit’ where the unit you just removed was before. Select that to choose your next team mate!