Issue migration dialog and avoiding custom field data loss due to changing context

Customers often don't realize that the data stored in custom fields with contexts restricted to an issue type or project will be un-retrievably lost when migrating to a different Issue Type or project. Although the migration dialog shows a summary of changes that will be made to the Issue, including a list of fields that will be removed by migrating, it appears this may not be visible enough.

1. I think it's arguable whether it's right for such custom field data to completely disappear from the database as well, because in most use cases a person moving an issue or migrating issue type is not necessarily intending to delete custom field data. Also, whereas other changes to issue fields can be mapped and even reverted in the future, data lost from the custom field cannot be reverted. Perhaps the data can be retained somewhere in the database even though it's no longer available in the new project or issuetype, but that's one of the issues.
2. Perhaps there should be some improvement in the migration dialog as well:

• can include a specific warning that the data lost can no longer be retrieved, which the user acknowledges to proceed, rather than simply listing the field in a general summary of changes confirmation screen.
• can include an option in the dialog to modify the context of the custom field to include the target project or issue type, which will automatically update the custom field configuration and avoid the data loss.

Workaround
See this knowledge base article for the necessary steps to change custom field contexts without losing existing data.