This might have been answered in a different post, but I cannot find it for the life of me, so here I ask: Is a skin a one-time use only, meaning if I used it on my weapon and I get an upgraded weapon, I cannot remove that skin and transfer it onto my new weapon? Or will I be able to remove that skin and use it as many times as I want?

You have a costuming tab of your inventory. They are equipped there and override the appearance of your actual equipment.

So if you bought a weapon one, it would be worn on your weapon costume slot and would change the appearance of any weapon you equip in your normal weapon slot.

In other words, you can use it for your character's lifetime. It doesnt get used up.

Oh, I see! Thank you :D

Costumes are reusable on multiple items, they are also easily swappable, its not like a remodel which in itself isnt all that complicated either, but a remodel is set on the gear part itself whereas a costume can be turned on and off an infinite number of times w/o the use of a NPC or any ingame currency.