As a world creator myself, I personally don't mind people editing my worlds to suit their needs. I just ask that no one alter any of the sculpting, though if an area isn't flat that needs to be etc, or a small hill taken down isn't a big deal. I don't even mind people editing the terrain paint etc like that. I would say if your just altering it for your personal use, and don't plan to share it or reupload and claim you did all the work you would be fine. You could also even credit the creator for their work, and link back to the original creator's post.
I have even edited other peoples work and reposted, most just ask that you don't claim as your own, and I never do, I will state if a world I post was made from scratch etc. So I really think your fine.