I see that the newer (2002+?) V6 engines have a newer tensioner. Is
this part an actual upgrade to the original unit, and can it be
retrofitted to motors with the older assembly?

Only the old assembly is available aftermarket from what I can see (on
NAPA), though there is a kit on ebay now and then with the belt and 3
idler/tensioner pulleys).

I think I understand the system, though it does seem pretty strange
(coming from many cars with simple hydraulic tensioners).