Originally Posted by Klonopin
Can't I downgrade? I thought that was a possibility?
One person has claimed it's possible through JTAG, but they declined to provide proof and nobody else has been able to do it.
So it is definitely not possible at this time.
If you want to run AOSP/etc you'll need to buy a used S4 with MDK (e.g. on swappa). They generally sell for a bit more than non-MDK ones, but I've seen them as low as $310-$335 recently. Lowest currently is $360 though. Basically, assuming your S4 is in mint/near-mint condition the price difference should be under $50 if you were to sell yours and buy one with MDK.