You are not supposed to change visa status whilst in the country on a non-immigrant visa. That being said, I'd agree with the previous poster that since you don't submit your passport to border control on departure so how would anyone know. Although you would need to submit you I-94 to show you have departed (this can also be done by post after leaving the country).
The opposite way round, entering on a C1-D and then desiring to remain in country for upto 6 months on the basis of your B1/B2 can be a lot more problematic and is most easily resolved by a trip across the border and back.