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Ex situ conversion of e-beam co-evaporated precursors was studied in an effort to fabricate high critical current YBCO films using the BaF2 process. The precursors were deposited on CeO2 buffered single crystal YSZ substrates. It was shown that an intermediate oxygenation annealing prior to the conversion modifies the precursor crystallinity and promotes c-axis epitaxial growth while randomly-oriented film formation is suppressed. With the modified precursors, a critical current density (Jc, 77 K & 0 T) of 2.1 MA/cm2 measured by SQUID magnetometry was obtained in 1.8 mum thick YBCO films. The mechanism of the pre-annealing effect was investigated by characterization of the precursors, quenched films, and fully converted films using XRD, SEM, and TEM. Cross-sectional TEM was used to study the early nucleation of YBCO film at the precursor/substrate interface. The significant effect of the precursor modification indicated that, in addition to optimizing conversion processing parameters, modifying the precursor is an effective way to achieve the desired epitaxial film structure and to obtain higher critical currents, Ic.