Good news for you, maybe no carepack required. If you are using standalone AP it is covered by Lifetime warranty with limited software support. Just go to HP Networking support website. Fill your part number of AP, login using Passport, then you will be forced to fill the serial number and then date of purchase.
https://h10145.www1.hpe.com/downloads/ProductsList.aspx?lang=&cc=&prodSeriesId=
But I studied an Iris application now (configuration tool for partners) and in enclosed carepack configurator there is a notice that Software support is limited to 3years only and Products purchased before August 1, 2013 include 1 year of technical support and 1 year (bug fix only). If you buy devices as new from official distributor no problem.
Carepacks can be bought only if you buy new device from official distribution channel. And there is a limit of 180 days after purchase until you need to decide if carepack yes or no. If warranty/carepack ends, you can buy postwarranty packs (90 days before warranty ends and 30 days after). Sure there are options to cover your devices by contract but this is more difficult.
Be carefull with versions. I am mainly using controlled APs and here first you must upgrade to lower version. (if I remember the key is: from older than 5.7.x first to 5.7, then to 6.0 and then to current version (6.5.3 is good candidate, 6.6.0 is too fresh and it is major release).
Read the release note.
Carepacks - if you buy from authorized distributor you can buy standard carepack U3SD0E (for J9427C), this is foundation care (software+hardware support)