I've got an older SATA drive (not sure if it's SATA 1 or 2). Same size plug as my new SATA 3 M/B.
Are the cables interchangeable? Any decrease in performance from using a "lower" cable?
Normally nothing has changed between the cables between SATA I,II and III. From the official SATA-IO document:
You can have a look at some benchmarks here.