In jdk15, JDK-8238048 moved the class file versions from the `InstanceKlass` into the `ConstantPool` and introduced a new method `ConstantPool::copy_fields(const ConstantPool* orig)` which copies not only the `source_file_name_index`, `generic_signature_index` and `has_dynamic_constant` from `orig` to the current `ConstantPool` object, but also the minor and major class file version.

This new `copy_fields()` method is now called during class redefinition (in `VM_RedefineClasses::merge_cp_and_rewrite()`) at places where previously only the `has_dynamic_constant` attribute was copied from the old to the new version of the class. If the new version of the class has a different class file version than the previous one, this new class file version will be overwritten with that of the class file version of the previous class. 

After `VM_RedefineClasses::merge_cp_and_rewrite()` has completed, we do another verification step to check the results of constant pool merging (in jdk15 this was controlled by `VerifyMergedCPBytecodes` which was on by default, in jdk16 and later this extra verification step only happens in debug builds). If the new class instance uses features which are not available for the class version of the previous class, this verification step will fail.