AFF actually runs a special version of ONTAP that only supports SSD aggregates, and is also licensed differently (Flash Essentials). You would buy a new AFF pair (which are being heavily promoted with specials, plus the new support options) and add them to your existing cluster, then you can move existing volumes or create new ones on the AFF nodes to take advantage of them. Alternately, you can just buy SSD shelves and add them to your existing 8040 pair, provided you can connect them in a new SAS loop. You won't get the full performance impact, and keep in mind that all those extra IOPS will tax the CPUs so tread carefully. Also, you'll need to upgrade ONTAP to 8.3 or newer to be able to create SSD-only aggregates (8.3 will only let you do it in CLI per https://kb.netapp.com/support/index?page=content&id=2021889&locale=en_US, but 8.3.1 adds support in System Manager for this). Of course you'd need to upgrade to add the AFF nodes into the cluster anyway.