REPLACEMENT

With Edit Mode turned on in OpenIV, navigate to 
GTAIV>pc>models>cdimages>componentpeds.img
and drag and drop the version of your choice.

ADDITIONAL DRAWABLE means without a script edit, you won't
be able to organically encounter Packie wearing his hoodie during
hangouts or missions, but you can use a trainer like ZMenu to
spawn Packie as a player model or a bodyguard and switch to 
uppr 002 (and hand 001)

REPLACE POLO SHIRT is what it says on the tin. While you'll see
his normal uppr 000 in cutscenes, in-game it'll be this hoodie.


MODLOADER

Simply use IVTweaker and my AddonPeds folder and just drag and
drop the version you want into the addonpeds folder, and instead of
you replacing an original game file, it'll just load the version in that
folder instead.

Same principal applies with Fusion Overloader, but I like IVTweaker
for its ability to load loose files :) 


Special Thanks to Al Alvarez for being my 3DS Max Jockey :)

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https://www.patreon.com/Brynna