If you want to change the room type that you've reserved, you'll need to call CRO.
If you don't really care whether you're able to upgrade or not, you can wait and ask at checkin.
Although some have been surprised with great deals, you need to expect to pay the full difference in rates between your reserved room and the room you wish to upgrade to. This can be more $$ than you would have paid had you booked the upgraded room (for instance, discount codes might have been available had you booked in advance, but might not be available at checkin.)
By waiting until check in to ask, you're also risking there being no availability for the room type you wish to upgrade to.
My philosophy is that you should always book the room that you want and that you'll be happy with.