Success story upgrading with miles from J to F at LHR this week. Had called a few days prior and was told that while most seats in F hadn’t sold, they weren’t able to upgrade more than 1 pax at that time. I was flying with my wife, so opted to hold out and wait to see if 2 seats opened up later.
Arrived at LHR about 3 hours prior to departure and asked the check in desk to see if upgrades were available. Given that LHR is outsourced to Menzies, the agent was able to add me to the list but said to check in at the lounge for a seat assignment.
Once I was through security and to the lounge, the EK lounge attendant informed me that seats still showed available, but they couldn’t confirm anything until 1 hour prior to departure. They said they would call me downstairs at the lounge if the upgrade cleared.
Sure enough, 1 hour until departure, I checked the Emirates app on my iPhone and noticed that our boarding pass had changed to F. Went downstairs and the agent seemed surprised, but was happy to print out our new boarding passes for F.
TLDR. Persistence pays off! Show up at the airport early, be kind to gate agents, sometimes things work out.