Originally Posted by
VegasGambler
You can pay cash for a flight and upgrade using miles. I did this when working for an employer who would only pay for premium economy on long hauls. Depending on the original booking code, it may require a cash copay as well as miles.
Whether or not the upgrade request will be cleared is another totally different question, especially, for general member and Premier Silver.
If OP's flight date is flexible, try to find flights with PZ space and get the upgrade request cleared immediately.