In the event that your employee has a job promotion whereby he/she is entitled to new leave entitlements henceforth, you can create a separate leave grade with the new leave entitlement. Here's how you can set-up a new leave grade:

However, if you switch leave grade/leave entitlements for your employee, existing leave balance will change retrospectively and the carry forward leave will change as well, depending on your carry forward leave settings in Annual Leave/Leave Grade. Before you switch to the new leave grade (with new leave entitlements), it is good to note down the final leave balance and update accordingly under Adjust Leave tab after switching. 

To adjust leave, you can refer to this link. 😃