You could split the group header into 2 sections. (Insert in the Section Expert)
So you've got group header 1a and group header 1b.
Format 1a as you would like it to appear before the drill-down, and 1b as you like it to appear after drill down.
In the section expert, select 1a and then click the "X-2" button to the right of the "Surpress" option.
Enter the following;
drilldowngrouplevel <> 0
Now select 1b in section expert, click "x-2" next to "Surpress" and enter this;
drilldowngrouplevel <> 1
That should get you want you need.
As always there's more than one way to skin a cat. You could achieve the same results with only one sections and multiple fields all with the same conditional surpression formulas. I just find it easier to work with multiple separated sections than loads of individual fields inside one section.
Regards,
Ryan.