The only way to do this with a single PS3 is to backup your saves onto media outside of the PS3. If Dragon Age 2 allows it, you could save them on a USB stick, swap the hard drives, and then copy them back over.
If the saves are encrypted, or won't allow you to transfer them onto a USB, your only option is to use PS Plus. Back up the saves to the cloud, swap the hard drives, and download the saves.
If you're using the same PS3, it's that easy. If you're moving the saves onto a different PS3, I believe uploading the saves to PS Plus will also get around that. You'll need to log into PSN in order for that to work, though. Using USB won't, though; savegames tend to be "signed" to the profile they were created with, and the system as well. So things get tricky if PS Plus isn't an option.