Change Description Label Based on Priority

import com.onresolve.jira.groovy.user.FieldBehaviours
import com.atlassian.jira.issue.priority.Priority
import groovy.transform.BaseScript

@BaseScript FieldBehaviours fieldBehaviours

if ((getFieldById(fieldChanged).value as Priority)?.name == 'Highest') {
    getFieldById('description')
        .setLabel('Why do you need this and why so important?')
        .setDescription('Please explain why this is Highest priority including details of outage etc.')
} else {
    getFieldById('description')
        .setLabel('Why do you need this?')
        .setDescription('Tell us why you want this.')
}