It cannot. The reverse is possible, but produces a brand new copy of the file converted over to the .FMP12 format.
What you can do is export data from one file into one of the standard text file formats and then import the data from that text file into the other. I use that method to update an old .fp7 copy of the known bugs list so that users with the older versions can still access the data with their older versions.
(note: but I will be discontinuing this practice in the near future.)