To move existing variants from one product to another, go to Inventory and select the product containing the relevant variant(s).
Select the variant(s) that you need to move.
Under bulk actions, select "Move to another Product".
Choose the product to transfer the variant(s) to. You will need to set up the new variant options.
Once you have set up your variant options, select Confirm. Your variants will now fall under the newly chosen product.