When you add a hard form to card(s) that already exist and have content, offer to copy the current content to a relative inclusion card (? if there are any of matching data type).
--John Abbe.....Thu Jan 31 07:47:39 PST 2008
Note that you can see the old content in the Changes tab.
Don't think we have design for this that we can move on. Availability via raw at least helps...
Migrate existing data when formatting existing cards+discussed in support tickets
Migrate existing data when formatting existing cards+relevant user stories