When you talk about substantive change, you mean change that really makes a difference. After a substantive discussion, you will have an in depth understanding of what you are talking about.
When something is substantive, there is a lot of there there, be it meaning or volume of things. The word brings a serious tone. While it is often used to talk about problems and their solutions, a big steak dinner could be called substantive as could a very long piece of writing. In any case, you use it when there is a lot of substance involved.
