How can we prove Plato's world of forms?

Do forms change in other objective worlds?

 

In the material of which they are composed.

The soul,, having once become in the form of a man has nothing higher to attain, and can never be degraded to anything lower.

Insight—A Record of Psychic Experiences—A Series of Questions and Answers, dealing with the World of Facts, the World of Ideals and the World of Realities Beyond Death