You can migrate and ASA 5512-X appliance with the old style IPS to one with the FirePOWER module.
You need to order the SSD and the necessary licenses. You can install the SSD in the existing appliances - it requires you power down first. The ASA will recognize it when it powers back up. You will then need to uninstall the IPS software module and install the FirePOWER software module
The FirePOWER module requires FireSIGHT Manager which runs on an external server - usually a VM for small deployments. FireSIGHT is available in 2, 10 or an unlimited (up to 25 for a VM-based installation) managed modules license levels. Those licenses are perpetual (although you pay for Smartnet to be entitled to support and updates). (NOTE - since originally posting this you now have the option of management via ASDM for Firepower on ASA. Firepower Management Center is still the recommended approach though as it offers more features.)
You will also need to add the licensing for the FirePOWER modules - a no cost Control license, an IPS license (optional but most customers order this) and an URL filtering and Malware (AMP) license. Those are available in 1, 3 or 5-year terms.
Your reseller should be able to help you with the necessary part numbers. It's a pretty common ordering scenario.
(Edited 2017-10-09 to update with current information.)