Actually I book my next cruise using the normal past guest option, if I decide to, I can still change it to early booking option,
here is the deal;
Early booking have a few limitations that I am not happy with for example,
if you find a better price in other place you can ask them to give you that price but it have to be in the way of proving it, you have to send them the add or online direction and I believe you even have to write them and send the actual prove.
In my case, past guest booking, it offers good prices and good deals and in the case I find a better price online I just call my carnival assitant, last time they still charge me the same price I got on my booking but they gave me a better category cabin, so being a past guest is good enough to get a good deal with carnival