Replacing Items

We have around 400 sales orders containing at least one of a certain product, let's call it Inventory Item 1. When this item is sold on the web store, it should include a free poster, Inventory Item 2. Instead of setting up a kit item on our store to combine the two, our manager made the mistake of creating Inventory Item 3, which he called "Inventory Item 1 - Web Store version." This has caused all kinds of headaches with our inventory management and fulfillment.

What I would like to do is somehow convert Inventory Item 3 into a kit item containing Inventory Item 1 and 2. This way we can consolidate our inventory numbers on Item 1. Is there a way to do this, such that it won't affect our 400 orders pending fulfillment for the item?