Changing the Existing Task Code Set on a Client-Matter

"If I change the Task Code Set assigned to a Matter, will the history associated with the original Task Set be deleted?"

No - changing the Task Code Set does not remove the historical data.  However, reprints of invoices generated under the old set will not display the proper Task Code Description and/or may display the message "Invalid Task Code".  In addition, you will be unable to see the historical data in Task Inquiry.  If you ever need to reprint an old invoice or view the history in Task Inquiry, you can change the Task Set back to the original, do what you have to do and then change it back to the new Task Code Set.