Can I replace the disk with an SSD easily? Probably not! :(
If it's the same as last year's model[2], it isn't that hard if you are used to working on laptops or mini-itx cases. Just have to be careful about dust, have a few torx bit screwdrivers, and some adhesive strips which you can find for $15-20.
[1] http://en.wikipedia.org/wiki/Fusion_Drive#Design [2] https://www.ifixit.com/Teardown/iMac+Intel+21.5-Inch+EMC+263...