RetailEdge does not allow you to change the matrix definition associated with a parent item once they have been created. We do this to prevent entirely different matrix definitions from being applied to the items. For instance if you applied a matrix definition with dimensions for Red, Blue, Green but then change it to Small, Medium, Large, there is no way for RetailEdge to automatically know how to apply the new dimensions with the old items (e.g., should the Red item become Small?).
However Mistakes happen and so if you need to change the matrix definition associated with a parent item, here is how you would do it.
If there are no children created for the Parent item
1. Rename the current Parent Item's Stock Number to something else (e.g., Stock Number-Old).
2. Create a new item with the desired Stock Number.
3. Go to Tools->Global Change->Inventory->Merge Items
4. Merge the old Parent Item into the new item from Step 2.
If there are matrix children
1. Rename the current Parent Item's Stock Number to something else (e.g., Parent Stock Number-Old).
2. Repeat the above for each of the Matrix Children items (e.g., Child Stock Number-Old).
3. Create a new item with the Stock Number desired.
4. Go to Tools->Global Change->Inventory->Merge Items
5. Merge all of the Child items into the new child items item from Step 2. (e.g, SKU-Red -> SKU-Small, etc.). This will move over the old item's sales histories to the new item and delete the old item.
6. Merge the Old Parent Item into the new Parent Item from Step 2.
You can also modify these steps to merge multiple items into one or matrix/sized items into non-matrix/non-sized items.
Either way you will be left with a single item with all of the history from all Parents and Children. and existing barcode labels printed under the old items will work with the new items.