Some users reported that using this FTF triggers an "update available" notification in PC Companion/Update Center/Sony Update Service. The firmware version remains the same (15.2.A.2.5), the only change is the Customization Version that updates from 1274-5443_R7A to version 1274-5443_R8A.
You can update through PC Companion/Sony Update Service to get rid of the notification, your personal data won't be deleted, but any modification made to /system partition will be wiped, so you'll need to reinstall anything that modifies the /system partition again (includes rooting, Xposed framework, bloatwares you removed, etc)...
Sony Xperia M dual [C2004]
ROM: 15.5.A.1.5 (stock)
Android version: 4.3 (rooted)
Bootloader: locked (unlockable)