By one time update, I guess you may not want to use options that involve perhaps a transition postfunction, that copies the values when a transition is executed?
There are a few other options that don't involve a transition post function at all:
For the post function method, you can use this plugin https://marketplace.atlassian.com/plugins/com.googlecode.jira-suite-utilities
If you're copying this field on multiple issues, however it will make sense to perform a bulk change for the transition of the issues, to copy the fields simultaneously.
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