No problem
Assuming that you're not flying imminently, I would keep your eyes on the FC upgrade price and see if it fluctuates.
I'm sure somebody will be along who can tell you what the mileage cost would be.
Finally, note that there has been extensive discussion on this board recently about the changes to FCM. You may be seeing a price online that is not the same as the difference between the FC fare and the fare you paid. However, the latter is still available by calling and a phone agent will be happy to price out that upgrade cost for you.
Enjoy your trip!
I'm glad I waited! Since it was a confirmed for less fare, I was able to get a first class upgrade for $138. Thank you!