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Use case
<todo>
Manual trigger
Create a new rule and name it appropriately.
Providing a description will help you to identify what the rule does but this step is optional.
Add a Trigger → Manual
No further configuration needed. The rule will only be executed if a user hits the Execute button 
.
JQL Selector
Add Selector → JQL Selector
JQL Query*
Use the following Expression:
reporter = nnnnn
nnnnn has to be the user name.
Update field action
Next to the JQL Selector click on Add → Action → Update field
Field*
Choose Reporter
Update to*
Choose Selected value
Value*
Select the new reporter that should be set.
Add comment action
Underneath the Update field Action click on Add → Action → Add comment
Comment text*
Due to an employee leave the reporter has been automatically changed.
JQL Selector
Underneath the first SQL Selector click on Add → Selector → JQL Selector
JQL Query*
Use the following Expression:
assignee = nnnnn
nnnnn has to be the user name.
Assign issue action
Next to the second JQL Selector click on Add → Action → Assign issue
Assignee*
Choose one of the given options that fit most for you.
Add comment action
Underneath the Assign issue action click on Add → Action → Add comment
Comment text*
Due to an employee leave the assignee has been automatically changed.
Screencast
This is how the configuration above should look on your screen
