Upgrade with miles question

I am thinking about upgrading to UCS from PE on my next flight. But at the moment I only have 17k miles on the clock, and I need 20k. Of course as soon as I fly i will pick up another 8k miles, which will put me back in the black.
The question is: Do VA allow to run up a debit balance on your FC Card, particularly if you are going to "pay them back" immediately?
The question is: Do VA allow to run up a debit balance on your FC Card, particularly if you are going to "pay them back" immediately?