If you have any questions, feel free to ask, I will gladly help. Setting up resorep is easier than it seems, and once
done, you can use any texture mod. I'm still thinking on a more clear
tutorial.
Resorep does not change the game, it is a directx hook that swaps the textures stored in the memory while the game runs, with
those you just chuck into a mod folder... Quite easy once it's set up,
and poses no danger or anything.

1.0 readme:

Disclaimer: I know someone already made a Duncan Valpole outfit on another site, but since I extracted the original texture myself, I can only credit the person who made me aware of RESOREP. Kudos forever... I would like to thank nfcintra as I first started to experiment with RESOREP after downloading some of his work, and understanding how those
mods operate.
The brown hair, eyes, and beard mod visible on these pictures are available on this site as well. This mod only includes the clothes.

This mod replaces the Politican's outfit (you can buy that one for 12000 since the start) with either the original Duncan Valpole outfit,  or with versions of something I tried to make out of it to resemble an actual British Royal Navy officer (lieutenant to be more precise) outfit. The navy ones have matching blue hoods aswell.
There are:
-Duncan Valpole outfit with original white belts.
-Duncan Valpole outfit new brown belt
-Royal Navy lieutenant with red sash (blue and red reminds me of later french colors....)
-Royal Navy lieutenant with bright blue sash
-Royal Navy lieutenant with blue sash.

if you would prefer the white belt for any Royal Navy outfits, just simply do not copy 1298750.dds to your mod folder, so the game uses the stock white belt...

A bit of history lesson: while Redcoats stationed on land, or redcoats carried on British royal navy ships are accurate, British ships should contain BLUE uniform navy officers and sailors as well. ESPECIALLY those who are situated by or near the wheel of the vessels- as only the army, and the marines wore red- Royal navy officers were wearing blue. (mostly the common navy sailors as well, though they had no official uniform by that time)

Current issues:
Stock DXt1 .dds files for politican's outfit, original Duncan valpole outfit and similars, has it's transparency/alpha broken BY STOCK. (You can check this if you examine the red sash on Valpole, it has black lines where we should see trough the sash. The same when you wear it...) So making other parts of this outfit transparent for further modding is not possible until someone gives me an idea how to fix an -originally broken- alpha channel....
Otehrwise, it looks cool.

Installation:

In the zip file I included all useful combinations of the files (folder name
tells you which one is which) decide which one you like, and copy all .dds files from THAT folder
to your RESOREP modded_textures_folder. (just the dds files, not any folder...) If you do not have Resorep, please continue reading...

Resorep installation:

Others written guides before me, but I made one as well, as straightforward as it can be in 7 easy steps:

https://www.nexusmods.com/assassinscreedivblackflag/mods/15

You can find all instructions there, but always feel free to comment and ask for help.