Hi

The "upgrades", as you called, are beyond Cosmetics and Software upgrades, at least as Yamaha concerns, there are new features, as an example:
In Tyros 5 vs Tyros 4 we find :

Audio link in Multipads (Not in T4)
Organ world (Also not in T4)
Ensamble Section (Not in T4)
New DSPs (Digital signal processors) This alone makes the instrument sound much better.
More S.A.2 Voices (Super Articulations Second Generation)
Audio Styles (Not in T4)
More sounds over all sections (Not in T4)
New Processors that make the instrument boot up much faster (Those Tyros2 users would remember, of course the ones with the optional memory loaded), and in other areas as well.
Etc.....

All this features can not be added through Software.

I am not up to date on the Korg models....so, somebody else may be able to clear that out too.