Default palette transparency index question

I’m trying to convert a large project to a new palette, but I’ve hit a large roadblock. The palette I’m converting to uses index 255 for it’s transparency, but aseprite defaults to index 0 and doesn’t let me change the index until AFTER the conversion from RGB into indexed. Because of this, the color on index 0 gets replaced with the wrong color across all 519 frames and 9 layers of my project. Is there anyway I can convert this file to the new palette without having to edit all those frames and without having to change index 255 to index 0? Ideally I’d want to change the default transparency index from 255 to 0 before the conversion, but I can’t seem to find a way to change the transparency index until after the conversion