Inheritance in unity editor question

So if I understood you correctly you have a base class that you attached to a gameobject and that is serialized and later you want to replace that component with a derived version? That's actually possible ^^.

A MonoBehaviour component in Unity represents several things at once. First of all there's of course the managed class you created. Besides that there's a native representation on the C++ side and that's also where the serialized data is stored. Whenever your scripts are recompiled, the managed side is completely destroyed and rebuild from the serialized data. You can actually exchange the managed class in the inspector. This can be done by switching the inspector into debug mode (rightclick the context menu at the top of the inspector and chose Debug). Now you should see the top field that is called "script" will become editable. It's actually the reference to the MonoScript instance / your script file that this component represents. You can simply drag a different MonoBehaviour script onto that field and the component would essentially "transform" into that new class. Keep in mind to switch the inspector back to normal.

However if you talk about changing the type of a custom serializable class that is serialized inline inside a MonoBehaviour, that's actually not really possible. First of all plain custom serializable classes inside MonoBehaviours do not support inheritance out-of-the-box. You can use the new SerializeReference attribute on a field. By doing this Unity does actually support inheritance and polymorphism, however it does not have any built-in way to specify the actual type you want to use. So you need to create some editor scripts to actually handle this.