Hi David, you are able to tweak things once a campaign is running, but I would advise doing so with caution. Since this can be hard to explain, I'm going to use an example flow...
1. Send email xyz
2. Wait 1 week
3. Send email abc
Once someone enters the program, they will immediately receive email xyz, then go into the 1 week wait step. Let's say that during that wait step you change email abc. When they're out of the wait step, the person will receive the newly approved version of email abc. Then they'll continue along the rest of the path as expected. This is the least dangerous/complicated update that you could make.
When you're talking about updating flow steps, you can do a lot more damage here. When someone is in a wait step, the number of the step is very important because once they exit that step, they will go to the next number. In the example above, if you added an email above 1. Send email xyz, then your new program would look like this:
1. Send email 123
2. Wait 1 week
3. Send email xyz
4. Wait 1 week
5. Send email abc
The issue with this is that someone who was in the first wait step above (step #2) is going to leave step 2 and go to step 3 in your program -- which in this case is send email xyz, which is the same email that they received a week ago. This will apply to any changes made to a program with people in a wait step.
Also, deactivating the campaign will not get people to stop flowing through the campaign. You will need to run a "Remove from flow" campaign if you decide that people should no longer receive the subsequent emails in a flow.
Hope this helps a little, best of luck with the first campaign!