Even one race on a new 12 sprocket could change the link spacing and stuff up the chain.
Remember you will be running sprockets with different wear factors, just think about what the chain does as it ramps up the two different tooth faces, it is goint to try to accomadate both sprockets, therefore put the chain to undue stress causing more stretch.
After 6 races on a 12 front sprocket, and then putting it back on to the original sprocket, could further stuff up the chain and the rear sprocket resulting in a broken chain and possibly a case.
My rule of thumb is never interchange chains and sprockets. Old chains do not ride well on new sprockets and vice versa.
A new set of everything will save you time and money in the long run.