Same thing happened to me. I just called FL Elite line, woman was very nice, put me on hold to "check with [her] supervisor," and returned to say that they would do it as a courtesy, but I would need to fly WN more this year to keep it (I had one painful RT in 2012).
I, too, asked if there was a certain number of flights I needed to take and she said, "No, not really. But one wouldn't be enough."

That's what I like- a firm plan!