You have the option to update any active module in your account, even after you've added it to a course and trainees have been invited to take it. Depending on the situation, there could be a few different consequences.
Trainees who have not started the module yet will see the updated module content.
Trainees who are in progress in the module will resume the module at the beginning of the updated content.
Trainees who have completed the module will not notice any changes. If you removed or added new modules to the course, a notification will appear on the real time reporting that they completed an old version of the course. No notification will appear if you just edited content within one of the modules.