If you are using BF Survey v2, the "My Requests" view allows users to view and edit their existing responses. In BF Survey Plus, it's called "My Surveys". To use this view, users must be logged in at the time of taking the survey.
If you wanted to hack the code, assuming you are using BF Survey v2, then the model in the front end for the view is where you'd want to tap into it, for example /components/com_bfsurvey/models/1results.php
There is an onBeforeSave function that you can manipulate the data before it is saved.
Just be aware that these front end files are generated dynamically, so any changes you make will be overwritten whenever you edit the questions in the backend. If you need to permanently change, you'd need to alter the file /administrator/components/com_bfsurvey/helpers/answertable.php
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