After I read the post saying blizzard is removing item-upgrading for now I gave the system some thought myself and I came up with a cool way to keep this feature while not making it necessary to upgrade all your items and giving something new and fun to do.
I think the item-upgrade feature could serve as an excellent scavanger hunt. It is used to upgrade legacy weapons (and later trinkets) into high level items. At the start of this feature there would be roughly 3 weapons per class that can be upgraded into high-level weapons all it requires is you to find the replica weapon dropping from all high level mobs in raids dungeons and zones or it to be dropped off a player in the pvp enviroment.
Once you've gotten the legacy weapon you can bring it to the item-upgrader and you will get the choice to turn it into either a pvp or a pve weapon. Depending on which you pick you will get 4 stage quests.
Stage 1 would involve buying/finding the materials needed to repair the weapon. This will be a mix of high level ores leather, dusts, herbs and gems.
In stage 2 you will be asked to kill mobs for a special reagent, this is some sort of legacy dust that will drop of all mobs in dungeons/looking for raids and players in battlegrounds + arena. Once you have this you can nearly use your weapon.
In stage 3 you will be asked to bring your weapon to (pve) 10 dungeons and kill the last boss + Will of the emperor + 2500 vp on any difficulty. As pvp you will be asked to win all battlegrounds twice + 2500 conquest points.
In stage 4 you will be able to complete your weapon, depending of which Ilevel you want for it you will be required to do rbg's arena's, bg's, lfr, normal mode or herioc mode to gather the last needed materials of various bosses + find the forge of legacy that is hidden in azeroth and changes location every day.
Now you have your legacy weapon completed to be used in the raiding enviroment. It will have the same stats, looks and effects as it did off old but updated if needed to.
I think this is a fun way to bring the upgrade feature back to life without making it necessary for everyone to use this.