Turast - You have the correct changes on 17, 1, and 13
12 You have the right spot, but the I doesn't change to an O
19 I don't understand how this letter change works myself, but both letters you have are wrong.
14 I believe you are substituting the correct letter but not to G.
These letter changes need to happen in EVERY across clue except for the 4 that contribute an extra word. So you need 12 changes in total.