Why can't it let you download the entire software and for people to disconnect from the internet, then uninstall the old McAfee and then install the new McAfee.
Because we found doing that, lots of people would reinstall that old saved package some time in the future and malware would get missed out because the definitions were way way out of date. The way we do it now is carefully calculated to give the best possible protection to the largest community of users.
There are 2 approaches to the issue
1- you could have directly started the installation of the new product while leaving the old product installed in the computer
The installer will remove the old product automatically and then install the new version.
2. There is also a tool called subdbfix.exe you can get it in service.mcafee.com kb id is 100966.
When you run this tool, it automatically changes your subscription from the old one to the new one, without uninstall and re-install
The process leaves Windows Firewall protecting you during the time that no software is installed and, unless you are surfing elsewhere or downloading stuff, there shouldn't be a risk. Most similar packages do the same, Norton and like.
The installation over the top mentioned by the previous poster should work also, however we have found that sometimes corruption can occur with that method, especially if it's a different suite altogether.
Subscription repair, if needed and in this case I don't think it is, is outlined HERE - consumers do not have access to the Enterprise and/or Service Portals' KB articles as he mentioned.