I was directed to this forum to find a answer for this question. Apparently this is the best place to ask for that and receive professional answer.
I wonder if 30.5 cm SK L/50 cannon could replace 28 cm SK L/45 cannons in Drh LC/1910 turret. I doubt that it would be feasible, so the second part of the question. Can Drh LC/1912 turret from "Derfflinger" be mounted on "Seydlitz" in place of Drh LC/1910 turret. Can it fit in to turret foundation? Or the size and extra weight make it impossible?
The 30.5 cm gun was much longer than the 28 cm gun (30.5 x 50 vs. 28 x 45). If you look at the turrets for these guns, you'll see that they are much different in size. So, the 30.5 cm gun cannot fit into the 28 cm turret.
Likewise, as the turret foundations and turret race diameter are very different between the two, the larger 30.5 cm turret cannot fit on the 28 cm foundation without massive changes.
Finally, as the 30.5 cm mounting was much heavier than the 28 cm turret, at best Seydlitz would have to become an 8 or 6-gun ship.