I have followed the first suggestion (using a CSV import) but I face a number of issues with this. First, it states that an empty Summary field must be imported in order to keep the original one. However, the import process does not allow me to import an empty summary field. Second the same Key should be used. However, from the list of available fields the "Key" field is not listed.
End result is that instead of updating the existing issue, a new one is created which obviously is not what I need. I'm using version 6.4 of Jira. What am I doing wrong??