Hi,
If you change the end value, only that will change, but the starting position will remain the same. You should create a new tween, so that it will re-take the starting position also.
About the "snap", if you mean that it doesn't rotate BY the amount you set, you should use the RotateMode.Beyond360 additional parameter, otherwise rotations will find the shortest route and follow that one.
Cheers,
Daniele