There are really dozens of ways upgrading Raid Weapons could be implemented in a reasonable way: 1.Upgrading the Raid weapons could reset its progress similar to old exotics 2.Upgrading a raid weapon could require a special item that only drops on VoG hard mode, similar how crux of crota will upgrade eidolon alley 3.Upgrading a raid weapon could require a special Raid bounty, like getting 500 Ultrakills with a VoC inside the Vault of Glass, or 500 Hobgoblinkills with A fatebringer, these bounties will drop randomly like the mentor missive from the bounty-bot, and only if you already posess the wepon needed for it