I have two users, A (admin) and B.
Logged in as B, I hit Win+PauseBreak, click Advanced, enter A's password, and choose "Adjust for best performance." I hit OK (or Apply and then OK) and OK and then close the remaining window. The taskbar now looks "old-style". Good. Log off and back on as B, and it's reverted my changes.
Logged in as A, it's already set to that and has been since several restarts ago. Apparently it doesn't apply equally to all users, and yet requires admin privileges. Fair enough, but how then should I change it for B such that it actually sticks between sessions?