If you have a chapter house, you can't change your EC. The property coordinator is a mandatory EC officer for chapters with housing (I think that even goes for chapters that have a suite in a residence hall, but I'm not sure). You can't just tack on another office to EC in addition to the ones that you have, and you can't, for example, boot the VPCR and stick the alumnae coordiator in there. In chapters that don't have housing, the 8th member of EC is the "Coordinator-at-Large," which is whichever coordinator is important enough to the chapter to be on EC, and that can be changed through some sort of voting process and approval by IHQ. All-knowing sisters, correct me if I'm wrong, because I don't have my notebook with me here at work.

If I remember, I'll go and look it up later and correct myself if necessary.
BTW, I'm the alumnae relations advisor for my chapter, so I definitely agree with you that AR is an important job!