I had my test at 8 or 9 AM and I was not an early bird at that time.
I did usually get up around 8 or so and study for a few hours and then show up to work around 11 AM (worked it out with my boss).
On the test day I had to get up at 7 and be at the test center by 8 to be seated by who knows when.
What I ended up doing was get up 2 days in a row before my test at 7 AM and make sure I kept the schedule. The day before the test I took GMAT Prep at the same time as I would real GMAT.
So, to answer your question - dont' think you need to change your schedule in any major way. If you know it - you will know it at any time as long as you are not too stressed out. It does help to start waking up at the same time as the test a few days before.
On the other hand, If you are not productive in the morning, don't think studying GMAT will make you productive. In case you are affected by morning in some significant way - try taking some tests in the morning and compare your results to those in the afternoon (you can use GMAT Score Estimator/Calculator to draw a common line between them).