There is no such feature. There are many structural differences between 6 and 11. Converting from 6 (simple) to 11 (complex) is one thing, trying to cram the genie back into the bottle to do the reverse would be quite difficult to do.
Best you can do is export the data from 11 into a tab, csv or merge file, then import that data into a version 6 file.
Thanks! Makes sense and your solution is a good one! Perhaps it is time I move to 11!