So, are u currently enrolled in a different program in BU? If so, you don't need to transfer.Can you clarify?
As far as I remember, they will start sending out the I-20s from mid April or so. If you have an urgency to procure the I-20, talk to the admissions office at BU and explain your situation. I-20 is not required to get the loan and is independent of the loan process eventhough it could be used as a supplementary evidence of your admission for the loan.
If you are leaving the country, clarify your transfer queries and also make sure to update your mailing address with BU.
Good luck!