yes, the WoW site is almost completely and utterly unclear with regards to being able to upgrade to a full account through the website itself. Everything talks about getting the free trial then buying a full retail key from your local store, which is what I had planned on doing just to find I could buy it through the account management section. You can go ahead after and upgrade to burning crusade aswell or you get a 10 day BC trial option once a full account.
ONly thing thats really irritating, no matter which way you go is that it takes up to 72 hours(and has taken over that for me) for the trial restrictions to be lifted, which is, bullcrap basically. I assume its to stop gold sellers getting trial accounts, passing them the money and passing it off again. As soon as you upgrade the level limit goes but you can't trade of use the AH. Buy the 20's without selling herbs/leathers/cloth on the AH its almost impossible to afford new skills. Just a bit of a pain.
I'm assuming that you get 30 days free, once thats up if you upgrade to BC you get another 30 days free. I'm not sure if you upgrade if you lose some of the first free 30 days or not. Will have to see I guess.