If you have filed I-485 then it would be very difficult to get the F1 approval. I-140 is immigrant petition, but opinion seems to differ on whether filing I-140 is considered showing immigrant intent. You can always file for F1, but it is unclear how one would answer the question in the I-539 form that asks regarding immigrant intent. A lot of people have asked this question, but no one seems to come back and provide feedback.
