There is no difference between new and updated issues in the handling of the text. You need to use quotes if the CSV format requires you to,
Regarding updating issues: In that case the CSV-file must contain the JIRA-Issue-Id column; keeping an empty string in Summary as "" cause the importer to wipe the existing text in the corresponding JIRA issue.
If you are using the CSV import file to create new issues then you need to put the correct text in the column for Summary. If however, you are using the CSV import file to update fields other than the summary then you should leave the summary empty.
Quote from Atlassian documentation on updating.
Importing a CSV to update existing issues will reset columns to their default values if they are not specified in the CSV.
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