Currently if you try a manual update in 4.8 it will probably tell you that it is up to date. The eventual update won't be automatic without your intervention, though I don't know exactly how they will do it.
Me I downloaded the English version and ran the off-line avast 5 installation file, with 4.8 still installed.
This will preserve the registration details, etc. of 4.8 and remove the 4.8 stuff too and install avast 5.0, this was a very fast installation. You will get asked if you want to use Google Chrome browser, if you already have it or don't want it check the No option.