I downloaded and installed over my 2015 patch C antivirus to see if that would work, and had no problems. After the restart it did come up disabled, then got enabled after a bit. I opened it and it had the databases never updated, so I started an update. It did a 50MB download. While it was downloading I noticed Defender was on and I couldn't get it off. But after the download it went away, and everything was as it should.