I would not count on the upgrade voucher clearing. Just take a walk over to the UA forum to see how often the GPUs do not clear.
If this is a "lifetime" trip and you will be totally bummed out if you are not upgraded then you should rethink the trip
The only way to be sure you are sitting up front in UA is miles or paying for the flight. Unfortunately too many of us very frequently have sat in coach on an expensive coach fare (you need W or higher fare) with another unused GPU.
Sorry to be a downer (realist)