From LightWiki

If you are currently using LightWave 8.0 and would like to upgrade to version 8.5 then no, you don't need to install 8.0.1, 8.2, 8.2.1 and 8.3 first. However, you will need to update your dongle drivers if you are moving from 8.0-8.2.1 to 8.3+. You can get the new drivers from Safenet. The same applies if you are still using 7.0 and would like to upgrade to 7.5d - there's no need to get 7.5, then 7.5b, 7.5c, 7.5d.

If however, you'd like to upgrade from 8.3 to 9.0? In that case, it will be a fresh install of 9.0. Don't install version 9 in the same directory as your already existing 8.3.