The latest version of LostHero have a very cool new feature now: you can quickly upgrade/downgrade items to the level of your character.
The upgrade will cost you gold and resources multiplied by the number of levels you want to upgrade by, and the cost is capped by 5 levels. So if you upgrade from level 90 to 105 it will cost you only 50 gold and 5x each of the resources require to upgrade 1 level.
Downgrade will cost you just gold, 10 per each level, and again to a maximum of 50.
Here is a screenshot from the upgrade/downgrade page in the game: