Source System in an ABAP Landscape

We are relatively new to SAP. Back when we initially architected the landscape we had many consultants in, including several Platinum consultants from SAP. At the time they emphasized the special nature of the "source system" in an ABAP landscape, and how it should never be refreshed and how we should not change which system in considered the source in our landscape. "Once the source always the source."

Now we are being asked to change the source system, but we do not have the background as to why the source system is so special and why it should not be refreshed or changed.

Would you be willing to share your opinion on the matter? Pros/Cons/Costs of Changing/Costs of Refreshing.

Thank you very much.