Ok, first of all This will only change the model displayed for you which means it is completly safe, GMs cannot detect and most likely will not care about you doing this so no worries :D
Warning: Make backups of any files you're going to edit. If something goes terribly wrong you will need to reinstall to get the origional models back.
If you don't already have the model viewer head over to [Only registered and activated users can see links. Click Here To Register...] even if you're not going to be swapping .dat files around its still fun to see what you would look like with full relic or hundred of millions of gil worth of armor (my RDM looks just right with excalibur :D )
Chose a model you want to edit, for example: The moogle.
Look through the npc models until you find the moogle, then note the number in the box in the top-right corner, (1-3-56.dat) notice how it is split into 3? well that is how we find the file.
Go to C:\Program Files\PlayOnline\SquareEnix\FINAL FANTASY XI or where ever your FFXI folder is.
We need to find the .dat (in this case 56.dat) in one of the folders here so take the first number (1) and open the folder for it.
1 = ROM
2 = ROM2
3 or 100 = ROM3
Now we need to find the sub folder for it, take the second number (3) and open the folder with the name same as that number.
You're in the folder with the .dats now find the last number (56) and rename it to something else but make sure you know which file it is incase you need to restore it.
Now use the same method to choose and locate the model you want to replace the other model with, in this example we'll use the taru archangel which is 2-22-11.dat
Copy the .dat (11.dat) then paste it in a different folder, change the name to the name of the .dat you wanted to change then cut and paste it into the folder where the 1st model was (remember to back it up!)
Congratulations you now have the TT ark angel as your moogle (or what ever else you changed).
If you want to change the .dat to a .dat you've downloaded it is the same method :P
Hope this was helpful, have fun.
Warning: Make backups of any files you're going to edit. If something goes terribly wrong you will need to reinstall to get the origional models back.
If you don't already have the model viewer head over to [Only registered and activated users can see links. Click Here To Register...] even if you're not going to be swapping .dat files around its still fun to see what you would look like with full relic or hundred of millions of gil worth of armor (my RDM looks just right with excalibur :D )
Chose a model you want to edit, for example: The moogle.
Look through the npc models until you find the moogle, then note the number in the box in the top-right corner, (1-3-56.dat) notice how it is split into 3? well that is how we find the file.
Go to C:\Program Files\PlayOnline\SquareEnix\FINAL FANTASY XI or where ever your FFXI folder is.
We need to find the .dat (in this case 56.dat) in one of the folders here so take the first number (1) and open the folder for it.
1 = ROM
2 = ROM2
3 or 100 = ROM3
Now we need to find the sub folder for it, take the second number (3) and open the folder with the name same as that number.
You're in the folder with the .dats now find the last number (56) and rename it to something else but make sure you know which file it is incase you need to restore it.
Now use the same method to choose and locate the model you want to replace the other model with, in this example we'll use the taru archangel which is 2-22-11.dat
Copy the .dat (11.dat) then paste it in a different folder, change the name to the name of the .dat you wanted to change then cut and paste it into the folder where the 1st model was (remember to back it up!)
Congratulations you now have the TT ark angel as your moogle (or what ever else you changed).
If you want to change the .dat to a .dat you've downloaded it is the same method :P
Hope this was helpful, have fun.