I think you'll need to call since they'll probably need to add the SIM card info for your Air manually as the iPad 2 wouldn't have an original SIM to switch from.
Another option might be to login to your prepaid account via a browser (instead of through settings) and see if it gives the option to "switch device" or something to that effect. Not sure if this is an option for iPad 2 (I don't see why it wouldn't be) but I know that it worked with my previous iPad 3 when switching to my mini.
Good luck!