I made changes to an existing map. The old save files don't reflect the changes.

The save file contains all the map information, so the map file itself is no longer required to continue the game from the save. However, for this reason, if you have changed something on the map, you need to start playing again from the main menu.
There is a save editor for "Heroes 3," you can find it online (although I have never used it myself).
In general, you should first provide a link to the save file where you are not able to collect the Cursed Armor; we could take a look and see what might be the problem. Are you playing it in HOTA or WOG? Or is it something simpler: you haven't removed an artifact that is occupying one of the slots where you need to equip a sword, shield, helmet, or ribs. If everything is in place, the game will offer to combine the assembled artifact, or you need to click on any component, and the game will display a description and a request to assemble the set.