EDIT: Thanks toAlexey Matveev the script is now working and all questions are solved
Below the edited script which works for JIRA 7.4.x Server
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.priority.Priority
import org.apache.log4j.Logger
import org.apache.log4j.Level
import com.atlassian.jira.event.type.EventDispatchOption
def currentIssue = (MutableIssue) issue
CustomFieldManager customFieldManager = ComponentAccessor.getCustomFieldManager()
//switch depending on the values of custom field and set priority accordingly
switch (customFieldManager.getCustomFieldObjectByName("Name of custom field").getValue(currentIssue).toString())
{
case "1. First value custom field":
currentIssue.setPriorityId(ComponentAccessor.getConstantsManager().getPriorities().find{it.getName().equals("Here goes the first priority name")}.getId());
break;
case "2. Second value custom field":
currentIssue.setPriorityId(ComponentAccessor.getConstantsManager().getPriorities().find{it.getName().equals("Here goes another priority name")}.getId());
break;
case "3. Third value custom field":
currentIssue.setPriorityId(ComponentAccessor.getConstantsManager().getPriorities().find{it.getName().equals("Here goes the last priority name")}.getId());
break;
default:
break;
}
// Update Issue
def user = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
ComponentAccessor.getIssueManager().updateIssue(user, issue, EventDispatchOption.ISSUE_UPDATED, false)