hi @ColdBuffalo ,
When we have already created a content fragment using content fragment model , then this warning comes in. Reason is to tell user that model you are trying to modify has already been referenced by content fragment.
In your scenario, you are trying to add a field in your model which is optional, so there won't be any affect to existing model. The field will start appearing in the existing content fragment created using same content fragment model.
Hope that clarifies your ask.