Data and response data will be saved, but will be archived (moved to another database table), if you deactivate the survey. If you are lucky (which depends on the changes made to the survey) you will be able to reimport those older responses to your survey again, when you activate it. LS will generally ask you about that. However, I don't think there is a guarantee.
Whenever you deactivate the survey, you should export the survey responses before. At least they are safe and you have them. If everything goes wrong, you can later join the two different tables manually in Excel. Usually this isn't a lot of work, you just need to find the new columns that you have inserted (or which ones you have deleted) and then adapt the two files accordingly. Generally a quick fix, if you don't make too many changes.
Of course, if you are not able to reimport the old responses into the system, you won't see them in the report/statistics.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
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