Product Attributes - adding additional attributes to existing product

I just added Size in addition to color in the product attributes. When I edit an existing product that has Color as a variation.....I cannot now add size as a variation. Do we need to delete the product and start over? Seems like you should be able to add more and more attibutes as time goes on???

What if you sell white shirts and then you start to sell colors of shirts....can't update a product later???

please advise

Mike