Good luck, you will be grinning from ear to ear.
The 600 will feel a lot more stable which inspires confidence. Try not to get nervous and keep a clear head!!
When my partner did her test she really couldn't get on with the 125, she was being pushed all over because of the weather, stuck her on the 600 and it transformed her riding confidence
Don't panic about the high speed stuff, I was told years ago the bike is better at straight lines than you, just relax and let it move about beneath you. Riding motocross teaches a lot about how they behave, to a beginner it can feel unnerving.