the 7700 refers to how many rpm's per volt the motor turns. On 2s the motor spins at 56980 rpm (7700kvx7.4v=56980rpm)
The 5700 on 5s would be 105450rpm (5700kvx18.5v=105450rpm)
that's way too much! You don't want to run the castle motors past 60,000rpm. You could go 3s on the 5700.
With the 5700 you would GAIN torque and lose speed. A lower kv motor has more torque than a similar higher kv motor.
The 5700 would still do what you want on 2s (wheelies at will) and run much cooler than the 7700.
I would sell off the 7700 and get a bigger motor(not castle).