The real issue with dataoperations like this is that LimeSurvey has no pause mode.
A pause mode which e.g let no new probands on the survey, but let the others finish their interviews.

The time gap between exporting and importing can cause change of data which is lost.

Currently there is only an importmode under "Create Entries".
Since there is an Export Button, I would favor an Import Button for the other import modes. Update and Replace. Replace overwrites everything with the CSV content. Update will only replace nonempty cells.
That way you can update data without overwriting everything else.

Thanks for the comment. The use-case that brought me here (my first survey): a mistake with the original Create->Import->.csv - where only the first 50 records had a valid firstname (the lastname field wasn't used) - the rest were populated with "O". So, in this case, I'd want the import/update to overwrite a non-empty field.

(Luckily, or by great design, the bounces suggest that the single character names were not used in the To header - they went as "To: invitee51@example.com" rather than "To: O invitee51@example.com").