Linux Lite 5.0 Final Released

[Moltke]

Is it possible to upgrade from 4.x or does it have to be a clean install?

As stated in the release announcement post

There is no upgrade path to or from RC releases.
There is no upgrade path from Series 2, 3 or 4 ( 2.0, 2.2, 2.4, 2.6, 2.8 | 3.0, 3.2, 3.4, 3.6, 3.8 | 4.0, 4.2, 4.4, 4.6, 4. 8 ) to Series 5.

So, no, it's not possible and yes, it has to be a full clean install.