Hmmm.. strange !! Im running 8.x I cannot speak for 7.2, I was able to re-ip a host from and objectgroup in asdm.. it did not keep the old IP, as seen bellow 10.10.10.1 was changed to 10.10.10.3.. I don't think there is an easier way other than let asdm do the reip, as asdm process will also update any acls or nat statement associated with hosts in network object groups.. you can do it manually through CLI but you realy need to be very careful in looking at the whole firewall picture acls as well as anything related to nat entries when changing host ips etc..etc..
object-group network OUtside_Vendor
network-object host 10.10.10.3
no network-object host 10.10.10.1
Rgds
Jorge
Jorge Rodriguez