The changes that cause the button code not to work are any changes that make the code different. This would mean any changes to variants and variables. Changing the price, or the name, or uploading a new file will not require a new code.
E-junkie does not currently have a way to notify you when you make a change that requires you to recopy and paste your button code to from E-junkie. I will pass this suggestion on to our development team.