Please - write in and compliment the crew! As cabin crew we do get notified of a compliment (not personal complaints - or at least i've never had one that I know of so I've never been notified!!) but we only receive them months afterwards due to internal processing.

A letter from a passenger complimenting the crew is GREATLY appreciated by any cabin crew member. Some of the things we have to deal with daily can get you frustrated with the reality within which we work. To start your day with a letter from a passenger can make all the difference.

When you write in, do not mention the "free" upgrade. And don't mention which cabin you were in - they can look up your assigned seat in the system if they need to.