Hello Fred,
Thanks very much for the interest in upgrading.
Once you've upgraded any given v5 licence to v6 you wouldn't be able to use that particular licence to run v5. (When upgrading a licence to v6 you would need to select a v5 licence to 'surrender'/exchange for the v6 licence.)
However, if you currently have two separate v5 licences and you upgraded one to v6 but not the other then the non-upgraded v5 licence would remain fully usable with v5. A v5 licence can coexist on a dongle with a separate v6 licence.
If you only have one v5 licence then there would be no need to retain the ability to run v5 anyway, since v6 is compatible with all platforms that could run v5, i.e. there are no changes in requirements between v5 and v6.