Most issue fields and custom fields can be merged with Merge Agent for Jira, under the Fields section.
Advanced content that can be merged:
You’ll see in parentheses next to these how many of a content type would be merged, e.g. “Attachments (3)” would show that we’ve detected 3 attachments that would be merged if the checkbox is activated. For comments and the work log, you can select that the original time the comment was created will be added to the comment as the first line (since the comments themselves will be shown as being created when you merge the issue).
Once you click the Merge button, you can see how we merge the fields and content into the other issues:
You may close the dialog at any time without the merging process being interrupted. Note however that if you start editing the issue right away, your changes may be overwritten until the merging process is finished.