If you install V.5.0 on top of 4.8 it will read the old license during the install, try it…
If that does not work you will need to do a clean install and register avast 5 online, and then there is no key
How to uninstall our software using aswClear.exe: http://www.avast.com/uninstall-utility
There are two methods of registering avast! Free Antivirus 5.0:
On-line registration
Off-line registration