@ResultLand I understand.
I am not sure about the level of success you will have with these suggestions, but they may be worth a try. Nevertheless, in the time that you vet these processes, it may be just as easy to re-create your entity in the new model.
(1) You could try deleting the new model ModelEntity from the Navigation pane, then rename your entity of interest to ModelEntity.
(2) From your old model, save a version which just contains the entity of interest (delete all other objects in the Navigation pane), then load the file as a library in the new model after deleting the default ModelEntity from the new model. Finally, use the subclass feature to subclass your entity of interest and use newly created entity (from the subclass feature of your entity of interest) in the model. Subclassing the entity will allow you to manipulate the entity object definition, as you would expect.
I hope this information helps. Thanks for using the forum.
Regards,
Alex