I also believe that you can move devices between plans as often as you like regardless of whether they are under a contract term or not and that this has no impact on the contract term length or existence for any device. I believe the only event that causes a contract term to occur or be extended is when a new device is purchased at a discounted price which requires one to enter into a new contract term or extension and that in that case the new term only applies to that particular device.
In other words, if you had an iPad to a shared plan you can take it out any time you'd like with no penalty.
I think the only possible negative of changing a device from one plan to another is that once you leave a plan that is no longer offered you can never go back. You can only change to a plan that is currently offered.