Ah, I thought it was someone in the refugee slums on Nar Shaddaa in TSL. Both being in K1 makes life a lot easier. It's just a simple override.
Open KOTOR Tool. The infected soldier should be:
Kotor I -> RIMs -> Modules -> tar_m04aa_s.rim - Undercity (Taris) -> Blueprint, Character -> tar04_republicso.utc
Double click the UTC to open it in the Creature Editor. Then simply change the appearance drop-down to one of the Republic soldier or officer appearance entries, like so:
Click the Save button to save the UTC somewhere. Then put it in your K1 Override folder.
Using djh269's directions, again in KOTOR Tool go to:
Kotor I -> RIMs -> Modules -> manm26aa_s.rim - Ahto West (Manaan) -> Blueprint, Character -> man26_manmerc.utc
As before, double click the UTC to open the Creature Editor and change the appearance drop-down as desired. Click save and put the UTC in your Override folder.
TWO THINGS TO NOTE:
Firstly, this will not work if you already have a save anywhere in the area containing the NPC. The appearance.2da reference is permanently stored in your save at that point, so a simple UTC override won't have any effect. You'll have to load a save prior to entering the area for the first time.
Secondly, doing the same thing for TSL is more difficult, as Obsidian was extremely lazy and did not give their UTCs unique names. Thus, you have to make use of MOD files. If you are using TSLRCM, as is almost certainly the case, you have to inject your modified UTC into their pre-existing MOD. That's a story for another day.