How to bulk copy value of custom field to same name's system field?

[Hisamitsu Koga]

I want to do bulk copy value of custom field (e.g. Due Date) to same name's system field (e.g. Due Date).

How can I do this? Could someone provide me with the detailed info, please?

[Pedro Felgueiras]

I think that function isn't available on Jira. 

What I would try is to make a script that iterate all the issue, read the value of the custom field and write it to the system field.