If the element has keyframes, the object can not be changed if the scrubber (vertical slider) in the timeline is not directly attached to any of the keyframes. If the scrubber is in the correct position, the object may be changed for this keyframe.
If the object is being changed, than this will effect the animation. The function "Apply changes to element" (1) must be activated. The timeline is then grayed out (2). Changes made to the object will now apply to the entire animation: