I see there is now a version 5 available. However, when I click on the Avast icon in my system tray for a program update, it tells me that my program is already up to
If you install over avast 4.8 pro it should retain your pro license details and convert into 5.0 pro.
If however you choose to do a clean install you will need to convert your 4.8 pro license to a 5.0 pro license file - avast! 5 Pro license convert from 4.8 Pro license. Try http://www.avast.com/resend-license.php now, fill in the form giving your email and 4.8 Pro License key and they will send the new license file to you (presumably it will check you have a legit 4.8 Pro license first).