Joel,
I don't have an answer to your query, but two experts have chimed in on that.
Your client is not the only one running 6.3.3. It works just fine for a small practice with simple charting needs like mine. And, the beauty of it is that one doesn't need to involve AC at all to move to or credential new machines. We moved to new Windows 10 machines last year without any problem. Couple 6.3.3 with an inexpensive, free standing ERx program and I am all set. So, depending on your client's needs, they may not have to change versions, hassle with AC, or pay for an annual plan.
And, Carl, hello! Hope you have made a good recovery from your accident.
And, James, I hope you have remained accident free!