Form of...
A human brain and limbic system encased in a gold bar!
.... yeah so it turns out that if you try to apply ... any even sort of consistent 'rule set' to shapeshifting... it results in either extreme body horror, or physics defying nonsense, or both.
I dunno, apparently caterpillars can somehow retain some memories when changing into butterflies, despite essentially turning into soup, so, maybe there is some kind of way this can make some kind of sense...