I believe it's 5$ to put a normal pre-order down, and 25$ for the special edition. So you would be able ot put the 5$ down, then just put down 20$ later. Or put the 25$ down now and just switch it later.
I went ahead and did the 25$ down, and asked if I could just call them when it's announced and have them switch it over that way. They said yeah, since the money is in the system. --- http://i52.tinypic.com/fxvu52.jpg
Forget that! Just order online, then, as soon as the CE is announced, cancel your regular and order the CE. You don't get charged AT ALL until it is shipped, but, as long as your order is in while they still have it available, they are obligated to fulfill it. You can even have it delivered on release day, or pick it up in-store.
Of course, Amazon is way more reliable than GS, and most big releases have release-date delivery for $0.99. When the CE is announced, I'll just cancel my regular and order the CE version. --- If you accept Shepard as your Commander and Savior, add this to your sig. WWSD: What Would Shepard Do? Founder - IRDC AND Fluffy the Friendly Deathclaw Fanclub