Personally, I would just change the original item. If you're looking to replace all instances of the item, modifying the original item is the best way since it would prevent cell edits and reduce conflict with other mods. Not to mention it could take a long time to find and replace each copy of the item in the game world. Typically, the fewer changes a mod makes the better.
I can understand not wanting to mess around with vanilla dialogue, but that's a whole different beast from item edits. When changing dialogue, you can inadvertently cause everyone in the game world to say the wrong thing if you don't make the changes correctly. However, any mod that changes the stats or behavior of items (modify cost, damage, weights, etc.) modify the original item. Many graphics mods also modify the original item, specifically if the mesh is also updated.
When you save your plugin the new values are specific to that plugin; you cannot easily modify an ESM which is where the original values are stored. Your changes will only be present if you load the game with your plugin; disabling your plugin will revert to the original values (or to the next mod in the load order that changes those values).