Power Armor has many different parts, so just switching one .nif with yours isn't probably going to work. Also I would recommend to make a separate power armor instead of replacing a vanilla one so you can avoid issues like this, if you have more mods installed, chances are another mod is overwriting your nifs with it's own
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I am looking for AC Odysset armor and outfits assets to port to FNV, if you know where I can get them, PM/email me.