Wrong point downgrade from Android 7.0 to Android 5.1.1 are done with special update.zip, witch can be use just for this scope.
How to downgrade from Android 7 to Android 5.1?Q5Pro: http://www.futeko.com/newforum/index.php?topic=1393.msg10921#msg10921
Q10Pro: http://www.futeko.com/newforum/index.php?topic=1392.msg10920#msg10920
Same for upgrade from Android 5.1.1 to Android 7.0 are done with special update.zip, name on this forum
STAGE 1.
STAGE 1. Migrate from Android 5.1 to Android 7Q5Pro: http://www.futeko.com/newforum/index.php?topic=1393.msg10919#msg10919
Q10Pro: http://www.futeko.com/newforum/index.php?topic=1392.msg10918#msg10918
Like tmihai20 write system partition is 800MB on Android 5.1.1 and 1000MB on Android 7.0, device info are in 20MB partition, migration from Android 5.1.1 to Android 7.0 or back request rebuilding all partition.
Safety way to do is to spend ~20 minutes and use special f/w build by HiMedia for this task. Installing Q5Pro f/w on Q10Pro is user mistake, they are lucky Q10/Q5Pro use some fastboot.img and box is NOT BRICK.
My point is that original Android 5 overwrites this but Android 7 does not. Strange at least.
I think it all goes down to needed space. Android 7 (Nougat) needs more space on /system partition. If you installed Nougat based firmware for the Q10 on Q5, then I would redo both steps needed for upgrading from Android 5.1.1 (Lollipop) to Android 7.0 (Nougat).