How to test new creatives without ruining the campaign?

It’s a very simple question, but I am getting super confused on the topic as there is different information since the andromeda update.

Very simply: if I have a campaign that is doing okay-ish, but its performance is dropping a bit and I think it’s connected to “ad fatigue”, what’s the rule of thumb now to add new ads/creaticez in? Copy the adset in same campaign and add new creatives in there? Just add the creatives in the adset? Or…?

I used to go by “never touch a winning campaign as you might ruin it, copy and paste it then change the creatives”, but not sure if this is still considered true?