That's an intended feature, the game does ask you if you are certain with your choice of battle class.
If you're really desperate to clear it, there is a workaround by using the 3DS versions of Spyro's Adventure or Giants, or the Wii version of Giants, due to a quirk with how these versions read figures, but it is not possible to do so through Imaginators itself as it's supposed to be a permanent choice.
As a software developer with 20+ years of experience this is the dumbest feature I have ever seen. Did you ever consider people would buy used crystals or one brother may give his to a younger brother? I just wasted $25 on. Dark crystal only to find out my son can't create anything on it.
So you're telling me that after my 6 & 7 year old children have spent their pocket money on these stupid things they are unable to reset them, as someone else mentioned these were brought second hand and with the game at the price of £22.00 (not to mention the countless amount spent on all the figures for this and the other games) whilst this may be much lower than it would be had they purchased it new it is still a lot of money to young children who now feel like they have been conned. which in turn means I have been conned as they get their money from me. I felt obligated to to write this post as i have just spent an hour comforting my six year old son who doesn't understand why he can't change his mind about the crystals set up.