I had participated in 6.5 beta program and tested that feature but I believe they have removed it from the final release and moved that feature to FMC Model Migration for 6.5 version. That allows you to change MGMT IP address with out deleting the FT...

only version 6.5 and later have the ability to change the FMC IP...you have to have delete the FTD from FMC first onearlier versions!!