Replacing button codes would be your only concern, since the button code for each product references the product's current Item Number. Our back-end system uses a secret Item ID number that does not change, so as long as you just edit your existing products (rather than deleting and replacing them with new products), everything else should continue working fine.
It would be a good idea to make this change when you're sure you won't have any orders coming through or pending payments waiting to clear in PayPal, since our system may not process an order correctly if it goes to checkout with any old item number(s) that you've changed by the time the payment is completed. You can even temporarily "close your store" while you make the changes -- in Seller Admin > Account Preferences, enter a future date in the "Make items unavailable for purchase until" field and click Submit, then when you're done, just go back in and delete that setting.
