As far as I know it is not possible to reorder shapes within a group using vba.
Also a shape inside a group cannot have a z order of 1 - are you sure the GROUP doesn't have a zorder of 1?
If you have a group with two groupitems at the back of a slide the Group Zorder will be 1 and the group items 2 & 3 The group items will never be 1
Try naming the groupitems (so you know which is which)
UNGROUP
get the z order of the shape to be removed
remove it and add the new shape
Move it's zoder to the removed shape order and then regroup