In FSX, I thought it was that once you reach a certain point, the ILS freq was entered into the ILS tuning panel, and you couldn't change it when it was done, but you could change it by entering it in the RAD section of the CDU.
I haven't flown the 767 in FSX for a long time, so I could be wrong. Plus I'm an old man now, so my memory isn't as good as it used to be. ;D