We would never know. This is all dependent on the Pixel Gun Company. At this point, from this upgrading system, it may not seem like skin modification when upgrading will be implemented, since every single weapon has a lot of upgrades. It could cost them tons and tons of money for cosmetic modifications for many weapons. Maybe more optional skins like the "Finishes" concept (by BastionMann) from the fanon wiki can be implemented, but who knows.