Abby Broyles of KFOR, who witnessed the execution, has this story. She quotes a statement by Gov. Mary Fallin that DOC was advised that the two substances were "medically interchangeable."
It makes sense that they would be. Both are potassium salts. When such a salt dissolves, the positive potassium ion separates from the negative ion. The purpose is to produce a massive increase of potassium ion in the blood, which causes cardiac arrest, and it should not matter what the negative ion is as long as it is nothing with a major biological effect of its own.
Sean Murphy of AP has this story, saying, "After the first drug was administered during Warner's execution, he said, 'My body is on fire.' But he showed no other obvious signs of distress." That is inconsistent with Broyles' report which said that Warner's statement was made before any of the drugs were injected. Both reporters were witnesses. Whichever version is correct, he certainly made his statement before receiving the potassium, the drug that would cause painful burning if the inmate is not first sedated.
Swapping another drug for the one in the protocol should not be done, and the state is correct to hold executions until this is all straightened out, but there is no reason to believe that the swap caused any actual problem in the Warner execution.