force priority lookup from client script

Now that ServiceNow is using a priority lookup table, does anyone know if it possible to force the calculation to be performed from a client script ?

I have an issue where (if the user doesn't have the right role) and they select an impact of "1" i am showing a field message and setting the impact field back to it's original value, however if they then select the same value again the priority lookup is not being performed - so I want to manually force the lookup to be happen.

thanks