Upgrades are usaully given out at check in as avaiable. It never hurts to call and make sure an upgrade request has been taged to your reservation. Most times I dont bother and when i dont get one I just ask. Works about 70% of the time. If your traveling with family, let them know you would like extra room.
According to the Marriott Rewards Platinum Elite terms and conditions you will be given the best available room at check-in but suites are specifically excluded (though they happen to be given sometimes). One exception is noted below where suite upgrades from studio and one bedroom may be possible. Here is the language: "When you check in, we’ll upgrade you to our best-available guest room at no additional charge. Not available for suites, Marriott Vacation Club International or Horizons by Marriott Vacation Club. For Residence Inn and TownPlace Suites, upgrades from studio to 1- or 2-bedrooms are not guaranteed." Frankly, it depends on the person checking you in, but more importantly on the occupancy load for that evening and the duration of your stay: if they are nearly full all bets for upgrades are off; if they are at a low occupancy then your chances improve. I agree with the other posters that it never hurts to ask, but sadly you can never assume that it will be automatic, and be firm but nice when you ask. Remember that associates deal with a lot of folks wanting something but who are pushy, and try to be the one they remember as being nice to them--it pays dividends!
Stepping Stone is spot on and another to add, my experience always included an upgrade at various hotels nationwide. Rule of thumb: Update in your profile that you would like early CI and late CO, but that you would like an upgrade, when available. That way, it is always there upon your reservation. Always good to ask for upgrade while they check on your reservation, you could have a new person (trainee) checking you in, which I have experienced many times. Congrats on your new horizon!, you will enjoy being near the top.