Any way to autopopulate a field upon transition if the field has a specific entry?

[Chuck Curtis]

When an issue is submitted, the priority comes in as Unset. We were wondering if there was anyway to change this entry upon transitioning the issue to the next status to a 3 if the status is still Unset? If a user selects a 1 or 2, we want that to stay the same. This isn't a make or break if it can't be done (as we're contemplating requiring a 1,2 or 3 entry) but didn't know if we had another option.

[Bob Swift OSS (Bob Swift Atlassian Apps)]

You can use the Update issues post function from Create on Transition for JIRA. You will need to use the conditioning support to only do the update for the conditions you have - see How to use pattern matching conditioning.