For whatever reason, the masterwork brave weapons that drop with the ornament (aka shiny) are treated as a different weapon than those that drop without the ornament and require an upgrade module. Non-shiny weapons can upgrade with just glimmer with the same non-shiny weapons. While I don't currently have two of the same shiny weapons to check, I expect that they would use glimmer to upgrade to each other as well.
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I kinda figured that was the case. All due to the ornament being tied to the weapon and not the account like all the other weapon ornaments. Still... I think the non shiny should be allowed to upgrade the shiny. It's like upgrading an exotic instead of a legendary.