When I was on 5.0.16 I had a user running 5.0.15's ZCO on Outlook 2007 and that was mostly fine -- is this still the case with a 5.0.16 ZCO and 5.0.18 server? We're not using any fancy features, just using ZCO to tap into the calendar.
That said, I forgot if I can merely run the MSI file on top of the existing ZCO. That won't force them to reconfigure outlook right? I'm pretty sure that's right but it's been a while since I had to deal with this.