Scenario 1 — A winning campaign
Do not touch a winning campaign. Don't add anything to it. Let it win. You're getting the results you want — maybe increase the budget slowly every three or four days and let it do its thing.
Scenario 2 — A campaign still in the learning phase
Don't touch this campaign either. Let it learn, and let it get out of the learning phase if possible. Don't reset the algorithm here — let it do its thing.
Scenario 3 — A campaign that used to perform but is declining
This is where you should be adding all your new ads. It's a campaign that did well in the past but has started to die down — CPA creeping up, ROAS going down, results not what they used to be. This is a great campaign to add new ads to and see if you can revamp it.
Scenario 4 — A campaign that never performed well
Here you can add pretty much whatever you want, even close it and launch a brand new campaign if you want. You have nothing to lose. Maybe the angle or the persona just doesn't work — feel free to do whatever you want with this one.