If you need to change a SKU, here is a step by step process that you should find useful.
Something to note before we get started.
Master SKU = The SKU found in the Master Listing
Market SKU = The SKU found in the marketplace listing.
We CANNOT change the SKU of a live listing. In order to change the SKU on the marketplace for a live listing, you will need to delete that listing.
Click on the SKU under the title to go the the Edit Master Listing for your SKU.
In the Edit Master Listing page, enter the new SKU in the SKU field.
In Listing Mirror, delete the listing off the channel whose SKU you want to change. Remember, you will need to have an active listing on at least one other channel in LM, so you will have the listing data to create from. This should preferably be Amazon, as it is the best channel to create listings from.
Recreate the listing on that channel.
That should do it! The created listing's Market SKU should now reflect the change you made to the listing's Master SKU. In addition, notice that the listing will now have "Alternate SKUs" listed. This refers to listings whose Market SKU varies from the Master SKU.
In the example below, sku2.5 is the changed Master SKU, while sku2 is the Market SKU used by the other listings.
Here are some other articles that relate to this topic.