FirstNo,
While I agree that you don't want to waste the CMs time with too much chattiness, I feel that you should personalize the letter a little. Maybe they do care it's your first trip to WDW and will make an effort to make it great! If it looks like you're just in it for an upgrade, then it could very likely end up in the "round file" with no action taken.
I forgot to add that you should include your name, dates and confirmation number prominently on the request. I bolded mine to make it easier to see.