Since you are afraid of causing jealous resentment and unwanted gossip "behind" your back, stay in coach, and take an upgrade on your private trip another time.

An alternative is to explain to your co-workers how clever you have been to accumulate miles to upgrade, and they will probably all want to find out more from you.

If it's me, I don't think I would upgrade if I'm the only one from the group. Just think of all the juicy office gossip that you will miss sitting up front by yourself.