Unfortunately, Google Ads doesn’t allow you to use a different Merchant Center account for an existing Shopping campaign but you have two choices.
Use a new Merchant Center
To receive the CSS benefits you will need a Merchant Center account under a CSS domain like Cobiro CSS. Then, you will have to create a new Shopping campaign and link it to your new CSS Merchant Center account. You can run both the existing and new Shopping campaigns simultaneously. Still, we recommend that you pause your old campaigns after the new ones pick up speed. When your Shopping campaign is configured with your CSS Merchant Center account it should be more cost-effective saving you up to 20% per click.
Use an existing Merchant Center
You now have the option to use your existing Merchant Center account by transferring it (Switch) to Cobrio CSS. This way, you will keep the link between your MC and campaign and the campaign will not stop running.
Note: Switching from a non-CSS account to a CSS account you need to be aware of what happens to other features such as Free listing. If a merchant likes to use Cobiro CSS for Free listing in Europe, the merchant will need to have a non-CSS merchant account (with a claimed domain) as well.
More info in the article Create a new MC account or use current one (switch)