Cannot downgrade. The %^&$ at Toyota Crap Entune support folks turn deaf when you ask that. 5.2 is a fail. Cannot pause, back, fast forward or start in the same place. I should have stayed with 3.2. Plus, the worthless tethering of phone to the car takes away from the clueless Bluetooth experience. This upgrade cannot suck enough. Toyota makes fine cars! Really, they do! They also make the crappiest Entune interface and do not support it. I recently called about a Bluetooth error and they were clueless. Next time, I'll replace with unit from Crutchfield.
There is indeed no way to downgrade to an earlier version of Entune. There's no technical reason for this, but some some reason the Entune developers deliberately decided to make a downgrade impossible. This is crazy; as a software developer myself, I *always* provide a downgrade option, because sometimes upgrades are problematic; that's just the way it is with these things. If nothing else, it should be possible to re-image the internal storage with a factory-fresh image, so as to reset the radio to the state it was in when it rolled out of the factory. This can be done with any computer ever made. But, no, they won't do it here. I just don't get it.