I'm in the process of in-place upgrading some of our older machines from 10.11 or 10.12 to High Sierra to prepare for the move to Mojave. The process is mostly complete, but after upgrading an old machine to 10.13, a "Previous System" folder is present at the root level. It takes up about 1Gb of storage on my test machine and doesn't seem to serve any purpose, so I'd like to remove it but for some reason I'm running into ownership issues. sudo rm -rvf, even when run as root, results in permission issues that prevent me from removing the folder. Basically a whole series of "Operation not permitted" errors on every single file. I've even set ownership of the folder manually, both in Finder and with chmod ops (not even 777 ownership on the individual files will work). The strange part is that I can manually drag&drop the folder into the Trash without any problem, but running from the command line doesn't seem to work at all. The only recommendations I can find online involve removing SIP temporarily, but I was hoping to avoid this if possible given the rest of the process is essentially finished.
Any suggestions would be greatly appreciated.