Suppose you offer a flexible plan. Users choose the features and you provide them a custom price based on the features they selected.
For this you have a product called flexible package.
Suppose a user starts its subscription with a custom checkout with custom price. This is possible at the moment.
Then they want to change their plan by removing or adding some features. Now you need to update their subscription object with custom price but this is not possible. You can only update it to another product/variant.