No drains on the late model torque converters. And on the late model Mustangs, Ford uses a synthetic automatic transmission fluid. They call it "lifetime". Yea, right.
The good transmission shops can use a low pressure flushing system to remove all the fluid and replace it with new. I've been using a local AAMCO shop for 15 years doing just this. My 2011 Mustang
had this service done twice; once at 32K, the second at 60K. It would have continued had I still owned it (sold it in my search for a Boss, or 2016 special edition model).
My 2007 F150 had 100K on it when I bought it used. Since I was unaware of its previous automatic tranmission maintenance, I took it to them. They did a free inspection, and gave it a green light for a flush. Most folks get weary on flushing high mileage transmissions that have never had service before, as they have read in the internet stories of a flush disloging crud in a transmission into the tiny spaces of the fluid plate. I have now driven the truck almost two thousand miles without a transmission issue, and the piece of mind I have knowing it has fresh fluid is well worth the $165.00 it costs me to do it.
If you want a complete fluid change, flushing is going to be your best bet, and doing it every 30 to 40K will be fine, with no adverse effects to the transmission in your case, since it is still considered "new".