First of all, Congratulation for passing BCECE examination. In Bihar Combined Entrance Competitive Examination (BCECE) whole process has include various steps like examination, counseling, seat allotment, document verification and then admission.
Merit list is prepared on the basis of the candidate's performance in the entrance examination.
Qualified candidates in the entrance examination, will be called for BCECE counseling for allotment of seats to various government, semi-government, and private institutions for admission in courses like Pharmacy, Health Science and Agricultural programs. As you have already received your seat allotment, now you will have to appear for document verification.
Here, all your documents will be matched with the information provided in the application form. You have mentioned that, during examination form fillup you did mistake and chemistry marks and biology marks got interchanged. But there is no change in total marks of PCB.
I think, during document verification when they will cross check your documents and information provided earlier in application form. There will be difference in subject marks only. There may be two things:
1. It may be possible they check only total marks and ignore your particular subject marks. If it will be like that then you need not to worry.
2. But in second case, it is also possible that they may notice the interchanged marks. Then you will have to be ready for convince the officials that it was done by mistake only. For this you will have to submit an affidavit stating "that this was done by mistake and my actual marks are so and so".
So, No need to worry. But again I suggest you, while form filling for any examination you should be very careful and recheck form before submission. Hope your query got resolved.