After a report has been modified with the designer the "group" options are no longer available on this tab. They only apply to the generator. The group options also may be changed in the designer.
Changing the sort order in the report does not change the report layout. If the first sort field is grouped (and may be a group header applies) there is a potential impact on the way the report will appear. Changing the first sort field may make the group header information meaningless. See "restrict data" for further discussions on this issue.