Assistance with customer survey - Can we modify a Survey and still keep all it's responses in the system?
You cannot edit a survey without first clearing it's responses.
I would recommend you make a copy of your current survey and add your new item to it, then when you are satisfied with the new survey you can "Start" it and "Stop" the original one. This should keep your old survey data intact and allow you to make adjustments to your Surveys.
Keep in mind you'll need to update your Survey Email template to refer to the new Survey Name or RecID as appropriate.