It is not uncommon that a person who was previously terminated is re-employed by the same company. If this is the case, you should NOT edit the previous agreement but use the previous agreement as a baseline by using the Re-Hire function.
By using the Re-hire function, you will treat them as a new employee under a new agreement. The personal details will be automatically copied, and therefore, you will only need to insert job-related details.
Previous Earnings
You can go to Tax Forms and download their FS3 to view their income and tax paid to date with your employment (see: Downloading Tax Forms). With this, you should add any emoluments and tax paid by any other job they had between the two periods of employment, if this is provided to you.
Inviting Employee
It is useful to re-invite the employee. Although the employee may use the same account without changing the password, they will now have access to each of their employment as different pages. This will ensure that any leave applications they submit will reflect the existing balance and the new agreement.