Form is the intelligible structure which makes a thing or being, giving it the character that the particular thing is. It is distinguished from Matter that embodies this structure. Also, Form makes Content as its important part.
Form in the abstract does not change as it is eternal, and the reason it is eternal because it has no Content. However, particular Form changes but those are called Content.
The eternal form of Idea of Plato has to change which concerned Hegel and in his Lectures on History of Philosophy where he sought to show that eternal form of the Idea has to manifest itself in different ways due to changes in history.