Back Liposuction for Wedding Dress Fit

There is no way to definitiively answer that.
Liposuction can and will reduce the upper back fat. I have found that ultrasonic assisted lipoplasty is useful in removing fat in this area.
However, the behavior of the skin is unpredictable and this garment may still gather loose skin in the upper back creating a crease. Overall, I would expect that the garment would not feel as tight.
There is no doubt that other techniques such as microcannula, traditional, power assisted, or laser assisted liposuction technques can be used and will be recommeded by other practitioners.
Clearly, on your workout regime, I would advise you to avoid any excercises that would build up your latissimus dorsi ("lats") because this will only make your garment feel tighter. Many personal trainers advise this because they want to build up your muscle tone without regard for how you clothes fit.

Liposuction of the back can give excellent results, and I have seen people who needed to fit into their wedding dress do just that!

It’s not unusual for patients to be concerned about
fat collections in the area beneath the bra strap. Unfortunately, the fat in
this area is extremely fibrous and for this reason, liposuction of the upper
back is difficult and unpredictable. This problem is further complicated when
excess skin is present as well. Under these circumstances, improvement can be
anticipated, but the majority of patients aren’t totally happy with their
result.

In some cases, alternate strategies have been
utilized. Recently, coolsculpting has shown promise as a method that freezes
fat cells in the upper back area.

Most of these approaches result in improvement, but
don’t totally correct the problem. They’re associated with pain, discomfort,
inconvenience, and expense. Under these circumstances, it
might be appropriate to alter your dress rather than expose yourself to the
downsides of liposuction for what might be a limited benefit.