Can I continue to use an Embedded IP Core in my design after my license has expired?
In short--Yes, this is possible. You can continue to use the license for the core in your project after the license has expired, that is, after the 12-month period.
There are 2 different aspects to IP Core licensing: the license agreement, and the license keys.
When you license an IP core, you get license keys that get enable a particular version of the core. That key is permanent, does not expire, and allows you to use that version of the core in perpetuity with no time limit in any design developed at the licensed site. In some cases, it might work across multiple ISE and EDK software releases as long as it is an Active and supported core.
Keeping the core in warranty enables you to access new core versions that are verified against new design tools releases.
The following link provides more details about IP Core licensing:
In summary, you can continue to use the license even if it is not in warranty. The warranty simply allows you access to updates to the core.