You can change anything that does not affect the structure of the database in an activated survey, like
change text of questions, subquestions, answer options
change equations in questions of this type (though this is nothing else than changing the question text)
change relevance equations,
change settings of questions
You can not
change the question type,
move questions to a different question group
add or delete questions,
...
If your changes take some time (more than a typo) you may deactivate it with the "expiry"option, After your changes just remove the expiry date.
So respondents who start the survey while you are changing the text of a few questions won't see the changed text of the first question but still the "old" text of the second.
Best you try it yourself.
Joffm
A bit more in the manual:
[url]
www.limesurvey.org/manual/Changing_an_active_survey
[/url]