The last time we upgraded, I purchased OBC to cover the difference in advance. When we could not upgrade with it bartender, I spoke to a beverage manager. I explained that we purchased OBC specifically to pay for the upgrade. If we couldn't upgrade, I'd be happy to have the OBC refunded to my card. Purchased OBC is fully refundable.
The choice was basically, take my money or give it back. I won't be offended either way. The bar manager confirmed that we purchased enough to covet the upgrade cost. Then he took the money.
I don't know if we would have had the same leverage without buying OBC. (We also had a bit of HAL OBC that was not refundable.)