If I deactivate the workflow and then reactivate it after making a few changes, will it restart the Automation for the previous leads, or will the leads be out of the Automation?

If you change the workflow, the behavior might vary for each lead because all leads won’t start the workflow at the same time and different leads will be in different steps based on when they started the workflow. If a lead hasn’t passed the newly added step, then the lead will be eligible for the step. If a lead already passed the modified step in the workflow, it won’t impact the lead.

Activating workflow of Automation by clicking Activate