Is it actually true that the 3-Phase AC induction motor can reverse as a generator!?! Don't you need an axial winding with excitation or permanent magnets around the core?!
Over to Dan Bentler: #1: YUP.
#2: NOPE. All motors can be generator and all generators can be motors. Paralleled alternators have reverse power controls to prevent them from becoming motors to prevent damage to prime mover.
One of the things you DO NOT do when switching from hi to low speed on motors is to do it quickly.
You should let motor stop and restart. The reason is especially on AC motors is that the motor is generating and can be out of phase - you will see the motor and load jerk when you get it wrong. Also doing this quickly the motor instantaneously goes from 3600 to 1800 RPM. - Dan