I don't think the task can modify the priority while the task is currently assigned, only at the time it is assigned.

I have a process that updates the priority by reassigning the form.

The user task is assigned with a due date that is 14 days after the instance start date, and the task has priority of Medium and a due date of 14 days after the instance state date.  The task includes a timer catch event that interrupts the task after 7 days. 

The timer catch event leads into another user task that assigns to the same user.  This user task has priority of High and a due date of 14 days after the instance state date.  This task includes a timer catch event that interrupts the task after 5 days.

The second timer catch event leads into another user task assigned to the same user plus their supervisor.  This user task has a priority of Urgent and a due date of 14 days after the instance state date.

The end result is that initially it shows Medium urgency and 14 days to due date.  Then on day 7 it shows High urgency and 7 days to due date.  Then on day 12 it shows Urgent urgency and 2 days to due date.

Here's what it looks like in the Process Model:

First User Task due date and priority:

Second User Task due date and priority:

Third User Task due date and priority:

First Timer Catch Event settings:

Second Timer Catch Event settings: