but then if a turtle crawls out of that egg it ceases being a chicken egg and becomes
I don't think I support this definition. It's too loose. You can't have it both ways. It is what it is when it is formed. It doesn't change after it has been laid. It's genetically primed from the DNA of its parents. Not from what's going on inside the egg.
Either way, I personally don't subscribe to the whole "chicken egg" definition issue at all. An egg is just the shell. The important part is what's inside it. And either way, both definitions can be easily answered by evolution, so it's never a difficult question.
If the definition is of importance, just ask for that info, then answer accordingly. No big deal. 👍