I was in the U.S. in 2013 for 170 days and 2014 for 160 days with a J1-PROFESSOR visa. Now I switched to F1 and receive scholarship from another university. The university tax system said I cannot claim tax treaty by filling out FORM W-8BEN. Is this because that I have become a Residence Alien status? So can I instead file a W-9 form to claim the treaty? Thanks.

The J-1 exemption period is normally two years, while the F-1 exemption is normally five years.

The J-1 period counts against the F-1 exemption, so you should still have three years of exemption.

You stated that you file as a resident alien. For which tax year did you file as a resident and wha was your visa status that year??