Welcome to FT! Also realize, that the 500 mile upgrades can only be used within your window of upgradeability e.g., 1K = 100 hours, PremEx = 72 hours, Prem = 24 hours. So if you definitely WANT to ensure you're upgraded, you can always use your miles to upgrade. You can also use miles to upgrade others as well if there is space available.
Do all of these responses answer your question?