We don't allow changing the optimisation goal for your campaign when it's already been launched. This is because each campaign is collecting unique learnings, and changing a strategy for an already running campaign would negatively affect its results.

Let's take a CPA goal as an example - it is optimising your campaign towards conversions, and if we were to change it to clicks (CPC goal), the AI would need to begin learning again.
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If you want to switch the optimisation model you need to pause, or stop the campaign - and create a new one with the desired optimisation goal.