Why am I getting an IncompatibleClassChangeError, and how do I slove it

I have a class that holds data: Book.
I needed to swap it to being an interface.
Easy enough - and now I have a SimpleBook class which implements Book.
I have a Jar that refers to Book Objects.
The jar was compiled when Book was not an interface.
even though Book has the same methods - I get an IncompatibleClassChangeError when Book is used in the precompiled Jar
Why is this?
and is there a way I can fix the problem?

the precompiled code only references Books and need
not know how to make them or that it is now actually
a SimpleBook.You have to recompile everything that uses Book.
I think calling a method on a interface uses a different bytecode to calling a method on a class, but I can not find it in the VM specs.
Edit: invokeinterface & invokespecial.

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Why am I getting an IncompatibleClassChangeError, and how do I slove it

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