Most traditional techniques for abdominoplasty (tummy tuck) tighten the muscular layer over the muscles in the upper and lower abdomen (we call this plication) as well as remove the lower loose skin (below the belly button) from the abdomen. Often, some liposuction is performed on the flanks and hips, but there are safety reasons as to why too much liposuction done in the upper abdomen can possibly affect the blood supply of the skin, for this reason many surgeons don't do a lot of thinning or liposuction at the time of the tummy tuck.
I tell all my patients that undergo a tummy tuck procedure that there is a possibility of a "2 stage procedure" with a 1 hour upper abdominal liposuction that I routinely schedule 3 months after the tummy tuck to further contour the upper abdomen.
I think it's great if you can lose weight with diet and exercise but even then not every patient will get contour improvement specifically in the upper abdomen (epigastrium). The good news is that you have come through the bigger part of the procedure, the tummy tuck, and even if you decide to have further contouring in the upper abdomen with liposuction to "bring your abdomen back into balance", it would likely be an outpatient procedure with a rapid recovery.
Hope this helps!