UpdateContentType is used in both Lists.asmx & Webs.asmx. Due to this overlooked bug or undocumented "feature", the call will always be directed to the Webs.asmx web service.
I was using SharePoint 2010 & jquery.SPServices-2013.02a.js when testing.
I was using SharePoint 2010 & jquery.SPServices-2013.02a.js when testing.