i think i have a feeling as to what might be going on.
I did a similar project with Ys Seven where i replacted the texture files in the USA version with the original HD chinese version ones, and found that falcom uses some kind of texture caching system that needs to be deleted before it actually falls back on and reads the .it3 texture files located in their respective folders - which is why even after replacing all the texture files... the original ones still show.
I have a feeling I found the files too - they are located in the ISO\PSP_GAME\USRDIR\data\cclm\map4 directory. labeled as .mc3 and .mc2 files
if I delete these and repack the game , when you load a level it will go into a "now loading" screen and stay there forever.
Now in the PC version of Ys i worked with, when these similiar files were deleted, it was smart enough to fall back on the "live" texture files in the .it3 files but it seems like the PSP is just dumb and doesn't have that functionality
i guess im SOL, but maybe this helps somebody in the future trying to swap texture .it3 files between different game versions/ports since it seems to be a running theme for how Falcom programs its games