"What" is a pronoun, but I would not call it a relative pronoun. It acts as the subject of the sentence and functions like "it" in an anticipatory usage.
Your change is not grammatical. Change "what" to "that".
Interested in Language
If I say: What caused his anger was the excess of authority. Is 'what' a relative pronoun that refers to 'the excess of authority'?
Can I change the order and say: It was the excess of authority what caused his anger? (using the anticipatory 'it'?
Thanks in advance.