As we mentioned it a few days ago, the Fortnite Upgrade Stations seemed to be inclined to evolve over the next few patches. This has been done with the new 11.40 update which brings a new feature to the item: the sidegrading. It has also seen a reduction in upgrade costs.
Now when you bring your Assault Rifle close to a workbench, you can modify it to become a Heavy Assault Rifle (or vice versa). This feature is designed to make the item interesting again, as it was rarely used at the time. It also seems to have been buffered, since the upgrade costs have been revised downwards. Count 200 of each material to transform your weapon from Epic to Legendary, compared to 350 before.
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The Upgrade Stations should be the subject of renewed interest, but it remains to be seen whether it will become really useful in-game.