The old IPS is, as you noted, completely unsupported. That is since earlier this year (April 2018). The announcement on that was made several years ago and customers have been encouraged to migrate to a new product.
At this point there are no migration promotions or special discounts. to continue IPS type coverage you should migrate to ASA with Firepower services (or possibly ASA with FTD image).
It's not merely a different subscription - you need to reimage the IPS module and install the Firepower (sfr) module and then setup that configuration. It's not super difficult but does require some basic knowledge of how the product works.