This is clearly mention in the prospectus that you will have to show all the original at the time of counselling.
If You will not able to produce OBC certificate, they will not consider you in General catagory. To make a rank list is not a daily process in IIT the is controlled by one of the IIT. At the time of counselling they are just varifing about you had mention in your form , that's it. So there is not any chance to transfer to general catagory.
In parallel of this, IIT system is very flexible also. They can give the time of 1 to 2 month only marksheet for those 12th student because may be possible some Board will not ready to produce marksheets. But not in case Caste certificate.
Main thing is that no of student will be behind you who want to get your seat. So in your case this is not possible to transfer in general rank.
So be careful in this matter.