Hi. I used DXTBmp to convert the files but as I said in my earlier reply it's not necessary. Remove the default textures with the same file prefixes before pasting in the replacement BMP files. (Back them up in case you decide to revert to the defaults.)
PPS: My download hasn't got any replacement files.
Airbus_A321.zip should contain 4 textures named as follows;
Airbus_A321_1_night_C.bmp
Airbus_A321_2_night_C.bmp
Airbus_A321_3_night_C.bmp
Airbus_A321_4_night_C.bmp
This is what the Texture folder of my A321 looks like with the replacement textures installed. I dragged the original night textures into the folder named Hide to back them up.