"the old icon must be attached to a real folder"... hmmm, to me all folders are real folders. Like "Prefs" or "System". Theyīre the original ones, created on OS3.9 installation.
It seems, these are the ones, that i canīt replace with new icons. That applies to some of my "Work:" folders, too.
New created drawers (the ones who get a green standard drawer icon) can be changed.
All executable programs, also.
Just some "original system folders" canīt.