User wants to change the employee for existing time and expense entries.
You can change the employee for existing time and expenses from the Time Entries or Expense Entries screen, respectively, provided you have not billed those entries. To do so, view the Time Entries (or Expense Entries) screen by project (Show > Project) and then change the employee from the drop-down in the grid for each entry. Alternatively, you can use the Batch Update feature to update the employee for the selected entries in one go. Check the Core Help for more on Batch Update in the Time Entries and Expense Entries screens.