A change in form is when something changes its physical appearance, but it retains its original nature. For example, a caterpillar changes into a butterfly. It changes its physical appearance drastically, but it is still the same creature with the same DNA and the same basic characteristics.
Change in nature
A change in nature is when something changes its essential characteristics. For example, if a person changes their religion or political beliefs, this is a change in nature. It is a fundamental change in who they are as a person.
Generally speaking, a change in form is less significant than a change in nature. A change in form does not change the essential characteristics of something, while a change in nature does.