After you edit the pre existing stadium, save and exit. You will see a photo of that same stadium. Enter the stadium again and it will ask you if you want to upload to vault YES/NO. As it's probably someone else's work choose NO. EXIT and it will save your edits. Don't know why, but it works for me.
Let's say you change the name of the Cornfield one to another name, let's say Ricefield. Once you do it it will ask if you want to save that change. Choose YES. When you exit you will still see the phot of the Cornfield. Enter it again, choose YES/NO to upload and then exit. Cornfield is now Ricefield.