To move one variant of a product from one parent product to another, first you will need to create the new parent product.
To do this simply follow the this guide to Creating a Variation Product up to the point where you have created the product and added the second variation attribute to the new product. You will now need to add as many variation attributes to this new product as you have on the product the child is originally associated with.
e.g, If your product has variants of both Size and Colour, then you will need to add these to your new product.You will need to ensure that you have exactly the same variation attributes on your new product as the old one otherwise the child product cannot be assigned to its new product.
Leave the initial child product in the newly created product as a place holder as this is needed in order for the product to be saved, but will need to be removed later.
Once you have added all of the new products you need, perform a full export of your products, making sure to export ALL Standard Fields, Custom Attributes and Variation Attributes.
Now in your spreadsheet you will need to edit the 'Parent Product SKU' column of your child product you want to move and change it to the SKU of your new parent product.
The below image shows what you would see in the product export from StoreFeeder before you have made any changes. You can see how the original products are associated with SKUA-Parent, and SKUB-Parent is the parent product that you have just created, with the placeholder variant associated with it.
In the next image, the products with SKUs SKU4, SKU5 and SKU6 have now been associated with the new parent product that has been created. This has been done by changing the Parent SKU of these products to that of the new parent, as circled. The place holder variant has also been deleted.If you use a phrase, such as placeholder, in the SKU of the placeholder product you can do a simple find and delete to remove these products easily.
Once you have made these changes, import the file back into StoreFeeder using the 'Product Import/Export' page and under the 'Update Existing' tab.
The image below shows how the child products have been reassigned to the new parent and is what you would see if you exported all of your products again.
Note:All the information relating to the product will also be moved, including inventory and variations.