What is the abstract noun for form?

There is not rule for changing nouns to abstract nouns. A noun is either a concrete noun or an abstract noun depending on what the Word means.

Concrete nouns are Words for things that can be experienced by one or more of the five senses; it can be seen, heard, smelled, tasted, or touched.

Abstract nouns are Words for things that can't be experienced by any of the five senses; they are Words for things that we know, understand, believe, or feel emotionally.

Sometimes concrete nouns are used in an abstract context, such as 'road' a concrete noun used as 'the road to paradise', an abstract context. But the Word road does not change, only it's use changes.