Currently, we can only replace one by one, when the comments option is selected in Writer find & replace dialog. This means that sanitising comments becomes very difficult as I noticed in bug 148333 (have to edit content.xml).
I support this enhancement request, but I'm not a member of design-team cc: Design-Team
Don't see a reason why Replace All shouldn't be available. The opposite is true from the UX POV.
*** Bug 148625 has been marked as a duplicate of this bug. ***
A missing piece of the implementation of a feature is a bug, rather than a potential for enhancement...