Although not recommended, you can restore your TA backup in any firmware version. Sometimes when you restore a backup taken in an old firmware version you get a bootloop that can be easily fixed by flashing the stock kernel again with FlashTool...
I'm saying that because I did something similar: before developing Nicki Root, I took a TA backup in rooted 4.2.2, unlocked bootloader, flashed 4.3 and rooted it by using CWM and then I restored TA backup I took in 4.2.2 and everything worked fine. I already did this procedure at least 5 times and it worked all of them, the only attempt I got a bootloop was after restoring TA backup while running a custom kernel, fixed by flashing stock kernel again
Another tip: create a TA backup with bootloader already unlocked too, if you ever need to unlock it again you can simply restore that backup, without needing to use fastboot and without wiping data partition
Sony Xperia M dual [C2004]
ROM: 15.5.A.1.5 (stock)
Android version: 4.3 (rooted)
Bootloader: locked (unlockable)