Spot Round is Different from other rounds in the following manner :
1. You have to pay fees before getting a seat.
2. If you get a seat and do not take the admission then your money won’t be refunded.
3. You have to drop any earlier allotted seat.
As for the question you asked, you can surely change your order of preference in the spot round. IIIT Allahabad is surely better than BIT Mesra. I think you should definitely go for spot round but at your own risk.