I'm afraid this only works if:
Before you load your game
for the first time after downloaded the HD texture pack DLC, you have to un-checking the .esp files under data files. Because, if you do not this, the new .bsa files (related with the DLC) will overwrite the "vanilla" .bsa files and the game will be "stuck" with the official HD texture pack DLC.
Am I wrong?

I don't know if this is true, HOWEVER, after installing the new HD pack ALL of my "loose files" (textures I have installed with Wrye using Bain archives) are being ignored. What is worse, if I uncheck and completely remove the BSA/ESP files for the HD pack from my game folder, it STILL ignores my custom textures (I verified they are still their, let Wrye re-index them, even re-installed them completely from their various Bain archives.)
It is very much like the game is now FORCING the use of the default textures, where it was forcing the use of the high-res textures, even after removing the pack.
Going to try and load an earlier save, but I would really like to know. How can I use this pack and still have my loose file texture replacements take priority. I realize it SHOULD work that way, but it doesn't.