Personalized content (based on something other than user credentials) and A/B types of content tests may only differ if you were using a manual approach previously. If you were using a third-party service, you can likely still use that service.
But if you were doing this work on the server, you have a couple options.
One option (usually the easier one) is to move to a third-party service. These often require adding some logic to your code, but the third-party can handle the trickiest bits, which is often worth the cost over continuing to manage the entire testing process.
Another option is to migrate your custom solution to one that integrates with your new composable platform. Most platforms offer the ability to hook into the request in some way, and manipulate the response.
This is likely to be different enough of an approach that it's likely more valuable to go with a third-party service.