Did you already pay the $1200/1800 for the Montgomery GI Bill? If so, then you will need to transfer to the post-9/11 GI Bill (a paperwork drill; should you so desire to do so).
Once you transfer over, then there is no turning back. Make sure that it is the intelligent decision for your circumstances. Generally speaking, if you are in a high-priced area (housing, and/or tuition), then the Post-9/11 Bill is probably for you. If not, then you may actually get more $$$$ out of the old GI Bill, which will put the money directly into your pocket.
I suggest that you crunch the numbers before making a irrevocable decision.
If you haven't done so already, consider doing the "$600 plus-up" on the Montgomery GI Bill. This will increase your monthly payments by $150 (IIRC).
Some quick figures:
Montgomery GI Bill pays me (starting Oct 1 when the new rates go into effect) $18216 per year, directly into my pocket. My total tuition over this past year (two regular terms, plus two summer terms) is right around $3300. That leaves $14916.
From what I can determine, the new Post-9/11 GI bill would pay my tuition bill, plus $1000 for books, and put $13476 into my pocket (the BAH portion of the payment).
Now, it is true that I have to pay for my books out of my own pocket with the old GI Bill, but that expense did not exceed $1000 (and I buy all my books from the overpriced campus bookstore instead of shopping around; lazy that way, I suppose). Regardless, I can write the cost of books off on my taxes (of which I suppose I will not owe a single penny this year).