I couldn't quite make out what your situation is.
(There is a difference between arrangine this *before* or *after* install.)
The key that is user is (like David said) RegisteredOwner in
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
When programs do something with the user identification, they will normally
take that info from there, upon installation that is.
However, once installed, most of the time they have their own registry
entries to record that, so basically they just copy.
Most of the time, because I recall from earlier times that eg. Office
installation used to take that info and incorporate into a (coded)file on
disk.
Obviously the registry entries can be changed after the fact, but you will
have to go through them one-by-one.
The file-situation would (AFAIK) require uninstall en install sequence after
having changed RegisteredOwner.
Furthermore, I would think it conceivable (not garanteed) that the
application checks for changes in it's registrykeys and starts acting up if
you've changed them behind it's back.
hth
george