For instance:
"I just downloaded some crazy awesome suit for Hoxton, but I want it to replace Houston's suit, or someone else's."

After you download the mod, just rename the files inside the mod:
(units/payday2/characters/shared_textures)
fps_sleeve_old_hoxton_df and suite_05_body_df, to the file names of the heister you would prefer the suit.

List of Heister Filenames:
Dallas = fps_sleeve_dallas_df & suite_03_body_df
Hoxton = fps_sleeve_old_hoxton_df & suite_05_body_df
Chains = fps_sleeve_chains_df & suite_02_body_df
Wolf = fps_sleeve_wolf_df & suite_04_body_df
Houston = fps_sleeve_hoxton_df & suite_01_body_df
John Wick = fps_sleeve_john_wick_df & suite_john_wick_body_df
Dragan = fps_sleeve_dragan_df & suite_dragan_body_df
Sokol = fps_sleeve_sokol_df & suite_06_body_df

As for the Lady heisters, Bonnie and Clyde Clover, it is a little more complex.
Bonnie's suit file directory is different from the other heisters.
Her suit file directory is [units/pd2_dlc_bonnie/characters/npc_criminal_bonnie].
So if you downloaded a mod for Clover, and you wanted it to replace Bonnie instead, you would need to add the file directory listed above, plus rename the files.

Female Filename List + Directories:
Bonnie's Suit = (units/pd2_dlc_bonnie/characters/npc_criminal_bonnie) npc_criminal_bonnie_suit_df
Bonnie's FPS Sleeve = (units/payday2/characters/shared_textures) fps_sleeve_female_2_df
Clover's Suit = (units/payday2/characters/shared_textures) npc_criminal_female_1_df
Clover's FPS Sleeve = (units/payday2/characters/shared_textures) fps_sleeve_female_1_df
That's not confusing, at all.

And as for Jiro... Jiro can pluck a duck.

That's all there is to it.
Not sure if this guide will come in handy or not.
But... Meh.

Cheers.