if I mirror a part in an assembly the new file references the original one.
Is it possible to change the reference to a different file out of that assembly?
There is no way to re-link the newly mirrored components to a different component or assembly. You can un-link the derived component from the parent. Once un-linked it would no long reflect any change to the parent component you mirror it from.
As you make be aware of any changes to the parent part is mirror over to the mirrored component (unless the link is broken) and any changes made to newly mirror component would only be reflect in that component and not the parent.
You could try breaking the link in the assembly and then doing a Replace from the browser to a different assembly but that kind of defeats the idea of mirroring the components in the first place. But you may have a reason to do so.