VA coded flight on EY in J purchased with Velocity points. Upgradeto F options?

[bnbmelb]

I managed to scrap enough Velocity points for 2 pax return to Munich on Etihad. Flights are VA coded. As this is the first time I get to experience a J product, I'm contemplating the option of upgrading the return leg from AUH to SYD to F. Has anyone done this before and what is the best way? Can I do it in the Lounge in AUH before boarding the flight, or try it a Check-in in MUC? I'm willing to pay if it's a reasonable amount. Thanks for any tips.