I'm surprised that you couldn't just update within the program itself... Generally, it's set to do this automatically; maybe someone who doesn't speak computer disabled the auto-update?
You could use something like housecall (
http://housecall.trendmicro.com/) to check for viruses before installing the new AV. If you're not comfortable using MacAfee anymore, you could go with AVG, which is free. I use the corporate version of MacAfee VirusScan and haven't had many problems with it, other than it blocking my email--but I have a university IT department who fixed that for me.
Before I had the free software through the uni, I used PC-Cilin and had no problems with it. It was way simpler to use and almost totally unobtrusive (except for the scheduled full-disk scans).
