When I copied a state machine from one animation bp to another (different skeletons so had to “recreate” the animation bp and it’s smachine), all went as expected - variables invalid, animations invalid, so I could manually replace them, but the “Custom Blend” graph between all the states that were using it, became read-only and I couldn’t find a way how to make it writable.
Ironically only thing that helped is bug/missing feature I have posted a few days ago - changing custom blend to standard blend and back to custom blend “fixed” the read only state of the “Custom Blend” graph for the price of removing everything inside it (not that big deal when one has to recreate the whole anim bp, event graph and smachines). I have noticed this bug in 4.7.5, no idea about the other versions.