Changing Sprites and Palettes

Hello,

So I'd like to add a challenging Monster-in-a-Box battle somewhere in World of Balance. To this end, I configured Enemy #1 (Soldier) to also serve the function of Enemy #377 (The Soldiers from Terra's flashback with Kefka), freeing slot #377 for a new encounter. The trouble now is that Terii Senshi's wonderful Sprite Editor no longer recognizes my ROM (nor does it recognize an untouched unheadered FF3US1.0 ROM, so perhaps this is a header issue?), so I have to find some other way to change the appearance of the sprite from a Soldier to something more suitable. I expect this would be rather straightforward, likely just the adjustment of a couple values, though if anyone could inform me as to where to look or what needs to be done, I would of course be most appreciative.

Thank you!