I know it may be not enough proof that the cockpit was moved back . So I have made an experimant and I merged three shots .The first one shows the Yak-9U armed with B-20 cannon ( red tone ) put on the shot of Yak-9D (B&W tone). The second one presents the same pic of Yak -9D but there was added the image of the Yak-9U armed with N-37 cannon ( red tone ). All pictures presented Yaks in almost the same position and weren't edited but just clipped and gave the same size. The only exception was the shot with the 9U with the standard B-20 cannon. I have had to slightly rotated it up in order to keep the correct side datum of both planes. As you may noticed, in both cases the 9U has the cockpit canopy repositioned back comparing to the 9D one. The difference is the length of the windscreen at its root. IMHO that's just the 40cm ( 400mm).
Yak-9D ...
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Yak-9D and Yak-9U with B-20 cannon
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Yak-9D and Yak-9U with the N-37 cannon
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