How to change my late husbands iPad to my Apple ID.

I suggest you do the steps in the article below to make it 'your' iPad.

Selling/Giving Away Device.  *

If you run into an Activation Lock, contact Apple. 

How to request access to a deceased family member's Apple accounts - Apple Support.    *

Apple ID - Official Apple Support.   -

Contact Apple for help with Apple ID account security - Apple Support.  *  (Link doesn’t always load)   

Get Support    -  Work through the screens to get a chat option. 

Apple Support schedule phone call.  -

Apple on Twitter-  

You are welcome. Since it is already under your AT&T account, I don't think the change will affect your AT&T status.