Hi PatrickS,
I'm sorry to say that the Search & Replace function only works for items in the Screen Editor for now. So if you open a display application the boxes won't be grayed out.
I believe it's because of some limitation within the CAD engine that is used for GUIDE, and as far as I know the developers are working on it.
This is something that could/should be stated in the manual.
Have a nice day
(Edit: acmall was quicker than me.)