dear student,

No, you can't. The only way to do it is to appear for NEET again, qualify, get a good rank and a take a medical seat after surrendering the dental seat.

I presume that your question hints at moving from BDS to MBBS like people move from electronics to computer science in engineering courses after first two semesters. Then answer is a definite NO. Can't do that, no way.

hope it helps!!!