Android 4.3 update got many changes and the kernel is incompatible, flashing 4.2 kernel while running 4.3 update result in your phone showing Xperia logo then freezing a cyan screen. Any specific reason of wanting 4.2 kernel with 4.3 update? Because this doesn't make any sense at all...
Sony Xperia M dual [C2004]
ROM: 15.5.A.1.5 (stock)
Android version: 4.3 (rooted)
Bootloader: locked (unlockable)