I asked my consultant about this after I read this soon after my first OGTT.
He said, that it wouldn't compromise the test, maybe he doesn't know, but I've always fasted and never upped my carbs! In fact my endocrinologist insisted I stay on very low carbs!
I think both arguments are perfectly valid.
Either eat LC as normal (as you intend to do for the rest of your life) and find out how you react to an ogtt under those circs,
Or
Eat carbs like everyone else, and produce a result that your medical professional can interpret according to their usual standard.
The important thing is to know that there are 2 options and make a conscious choice.