Generally if at all possible you don't want to modify emails that are live while leads are in the flow of a campaign using it. However the latest approved email version is always sent out, so as long as your well ahead of a lead passing through you should be fine.
Modifying steps in a campaign typically are a challenge when modifying wait steps, The waitstep setting at the time the lead entered will stay in effect. So you'll need to be aware, if modifying waitsteps while leads are in the flow