You don't have to worry about avast home as the install over the top would retain it, or if you do a clean install to avast 5.x you can register from the avastUI, Maintenance, Registration section. This way there is no wait for an email, it should show the product registration details in the avastUI.
Sorry you have me confused, easily done.
Installing what 3rd party software, you said your other avast 4.8 pro versions are server managed, so my assumption is that is the avast ADNM/SBS versions.
The process I mentioned above is for stand alone installations. You asked about the managed clients.