How secure is information encrypted in OS 10.7 now (October 2016)?

How secure is information encrypted in OS 10.7 now (October 2016)? I have portable (flash) disks encrypted in 256 bit in OS 10.7. How secure is the data now? Is it easily accessed? Should I unencrypted them and re-encrypt them on my OS 10.12 (Sierra) iMac for better security? Thanks.