I would do it every 40,000 miles, drain and fill only.  Right now you are overdue.

Are you putting an average of 7,400 miles a year on it?  Then that would translate to about every 6 years to change the fluid out.

It's in your "Scheduled Maintenance Guide” or “Owner’s Manual Supplement”. Hopefully you still have it. If I remember, for normal driving 45-60k first one and then every 40k miles. We've all seen Scotty's view on life of the transmission fluid, and I agree there's no such thing as lifetime fluid for anything. Drain it, measure how much came out and put that amount back in. 

It's been 10 years now.  I would absolutely change the fluid and filter now and at least every 50,000 miles.  Would you ever change your engine oil without changing the filter?  I would not.