Hi, David!
Unfortunately, no. You need to have the build earned before you can buy that new class in the first place. Not all of the build need be spent before purchasing the class, but you do need the initial amount.
The Character Builder lets you make a brand new character. Changes need to be worked out on the bottom-left corner of the character sheet following the rules regarding learning or unlearning skills. As a member of the Card Team, I'd be glad to help explain further if needed.
Feel free to send me a PM either here or on Facebook and I should be able to solve the build problem and hopefully others!
-Jeff
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OOG -- Jeff Balla
Card Team
Kitchen Staff