Where do you set/change the SKU for a new/existing product? - Drupal 7 Commerce i can't seem to find where to change the sku for an existing product variation. is this even possible? Where do i set this?
You can change the product SKU on the product edit form.

e.g:

admin/commerce/products/1/edit

Because products are referenced on line-items etc.. by their ID value, it's actually OK to change SKU for existing products if needed.