I'm sooooo not the person to ask! My instructions every time have included a three-day carb binge before the test. Both for the two I did while pregnant and the two I did while not...

But I imagine it's not going to do any harm to change your diet in that way, so you could easily start with gradual changes now so that it becomes a normal thing and not just a change to prepare for the test that may become undone later.

BW