absence management has 2 parts:
a, substitution: Sarah handles work of Thomas whenever he is on holiday
b, absence: date when the employee (e.g. Thomas) is on holiday
if both of these are set, then all tasks of Thomas are available for Sarah to handle them as if all of them would belong to her.
For these reasons (new) tasks do not have to be re-assigned. It happens automatically.
Please take a look at latest release of Axon.ivy Engine with Firefox or IE. Deploying JSF WorkflowUI during the creation of a new application will enable you to set up these absences.features. It's a good idea to define an admin (permissions). She can even set absences for other people.