Unfortunately, it isn't possible to change the lab on an existing pricelist because each lab offers different items. However, you can select a different lab by creating a new pricelist.
Select your new lab from the Choose Your Lab drop-down, then finish the process of creating your pricelist.
Don't forget to apply your new pricelist to your site by checking the Make this my default pricelist checkbox before you save and exit the pricelist screen.
Note: Setting a default pricelist is an option only available on the Pro plan.
You'll now see the products from your newly chosen lab available within your galleries instead of the items from your old lab.
Note: To view the custom pricing as a guest would see, log out of your SmugMug account or view your site in a private browser window. While logged in you'll always see the base pricing and all products, not your customized pricelist.