I installed Windows 7 via bootcamp several years ago with much older version of macOS but now when I want to delete it via Boot Camp Assistant, it doesn't recognize my bootcamp to remove and also tells me I need to have my whole partition as one, which makes sense.
I read around and can try to remove my Bootcamp partition via Disk Utility but I don't think that would remove the Bootcamp menu and because I'm planning to install Windows 10, I'm not sure how things will go since I have already "a bootcamp" configuration.
Thanks.
I ended up deleting both newly and the Bootcamp via diskutil and trying to install Windows 10 via bootcamp again and I'm facing new problems that I think is better off with a new question than continuing in the comments.
– idn Mar 22 '19 at 00:19