This is both a migration and a "oopsy" correction problem. If an idea is created in one product and needs to be moved to another, the custom field data is lost, even when fields are mapped identically. This makes it impossible to correct ideas accidentally put in one product; and also prevents us from carrying ideas from an "old" product are to a new product area.
As long as the custom field name and key is identical, the data will carry forward to the new product