IMPORTANT NOTE: For the latest UI DAT updates for Final Fantasy XI, please see Emiko's FFXIStyle.
Back when I was still playing FFXI and updating the UI DAT files, I wrote myself a little program called iconswap to make it easier for me to update the icon dat files every patch. It is a textmode program. To run it, either make a batch file or run it from the command prompt in Windows. Here is the batch file I used for the last UI update I did last year:
iconswap 57.DAT ROM\119\57.DAT.NOV28 ROM\119\57.DAT
iconswap 57-SE-utsusemi.DAT ROM\119\57.DAT.NOV28 ROM\119\57-se-utsusemi.DAT
iconswap 57.DAT ROM\0\12.DAT.NOV28 ROM\0\12.DAT
iconswap 57-SE-utsusemi.DAT ROM\0\12.DAT.NOV28 ROM\0\12-se-utsusemi.DAT
iconswap 57.DAT ROM\176\107.DAT.NOV28 ROM\176\107.DAT
iconswap 57-SE-utsusemi.DAT ROM\176\107.DAT.NOV28 ROM\176\107-se-utsusemi.DAT
iconswap 57.DAT ROM\178\46.DAT.NOV28 ROM\178\46.DAT
iconswap 57-SE-utsusemi.DAT ROM\178\46.DAT.NOV28 ROM\178\46-se-utsusemi.DAT
The first file in the command, "57.DAT", should contain all your latest custom icon graphics. The next file, "ROM\119\57.DAT.NOV28" should be the latest UI dat file from SE. The last file "ROM\119\57.DAT" is the file that will be created by running iconswap. I have a command for each language UI file and also for versions using the alternate utsusemi graphic. So I ran iconswap in a directory with my source 57.DAT file in it, and then subdirectories for ROM\119, ROM\0, ROM\176, and ROM\178.
Note, you may need to have Cygwin installed in order to run my program. I only wrote it for my own use and I may have deleted my source code. I never expected to be giving it to anyone, so there is a high likelihood that it won't work at all, or won't work for you. Also, if SE changed the icon dat file format since I last played, then iconswap may not work properly anymore. If SE is still using the same file formats, then the only work you still have to do is to get all of the icon graphics you want in the right locations into your source "57.DAT" file. That requires some understanding of hex editing and the icon formats, which I don't plan on explaining. If you know where the icon graphics should be, you just have to copy any new icon graphics from a new SE dat file into your pre-patch 57.DAT file, then rebuild. The whole thing usually took me 5-10 minutes after a new patch was released.
I haven't played FFXI in a while and won't be returning so this program will be unsupported forever.Download iconswap here: iconswap.zip.