You'll need to contact Verizon to activate the tether function on your phone. It'll be more money (of course), but that's all you can do.
And, as far as I know, the iPad doesn't care where it gets WiFi signal from, be it a phone, computer or modem/router. It'll just find the signal and request access (if password protected). So, it doesn't matter if the phone is Verizon and the iPad [was] AT&T.
If your iPad is 3G and you currently have an active plan running, just cancel it (via the iPad, online or by calling AT&T <---which is the last option should the first two don't work out). The AT&T plan for the iPad is a 30-day plan that you can cancel at any time without penalty. Once cancelled, the time that you've paid for will still be there until the 30 days is up. Then, it won't renew. Of course, if you haven't activated the 3G (or it's WiFi only iPad), then there is nothing you need to do. The iPad will find the signal put out by the phone...
The key is that you must initiate the cancelling of the iPad plan. It'll automatically renew until/unless you cancel it.
Hope this helps.