I'm thinking about using Lime Survey. I will occasionally have the need to reset the expiry date after a survey has expired. Is this possible? If so, can it be done for a specific survey responder?

You can always change the expiry date, even for active surveys.

For specific participants you can set a token valid from/to date when using closed surveys.

If someone has submitted a survey, can you allow them back in to update one or more responses? If so, do you track the different submissions?

For open surveys users have to click the "save and resume later" button to be able to re-enter the survey.

When using tokens your survey has to be set to non-anonymous and you have to enable "token based answer persistance" at survey settings -> tokens to let users re-enter the survey and change their data set.
If you set this differently, a new data set may be created instead of re-loading the old data set for editing.

Is n't it so that, if a survey is expired, setting the expiration date of the token does'nt have any effect? I tried it, and it appears to work like that.
Frans, DigiPsy

fransmarcelissen wrote: Is n't it so that, if a survey is expired, setting the expiration date of the token does'nt have any effect? I tried it, and it appears to work like that.

That's what I assume as well. "Closing" the survey by setting an expiry date closes the survey for everyone, even though their token may still be valid.