If you are currently silver I would expect you to upgrade to gold directly from the moment you reach the required number of miles. Which can happen by either flying, or by purchasing those 500 miles. I would also expect your year to reset at that moment, meaning that any flights taken after that moment will count for next years requalification.
If you would fly 8K miles now (while you only need <400) may mean that the other 7600 miles do not count for requalification, though I am not exactly sure about this.
I would expect that if you purchase the 500 miles, that you would be upgraded now, and can enjoy the benefits on your next trip. That also means that those 8K miles with your next trip would count for re-qulification.
This is how I understand the rules at least, maybe somebody else with more experience can confirm
