Nice answer, Debra!
I have one additon to add.....
When ICD-10 becomes effective, ICD-9 may still be used by worker's compensation and possibly by some auto insurance carriers. WC and auto currently use CPT codes (and I would imagine ICD-9) that are several years old.
I think this creates problems for those billing WC or auto, but unfortunately, if you accept them, you have to bill them at their direction.