Although I can't know for sure, I think it would not have been possible. The 305mm guns weigh 20% more than the 280mm ones and need much stronger cradles to handle recoil; I doubt they could be crammed into a 280mm turret. The complete 305mm turrets had larger barbettes than the 280mm, so the whole ship would have to be gutted to fit them (either the guns need to be moved closer to the ship's ends or the machinery and Boiler spaces need to be shortened to retain the length of the citadel, or a mix of both measures); although this seems abstractly feasible given that all German WWI era ships were underarmed for their size and had ample weight reserves, it would certainly be prohibitively expensive and not worth the effort.
The examples for up-gunning battleships included a reduction in the number of guns per turret, so the new turret would fit on the diameter of the old barbette (which of course would have to be totally refitted internally with all new elevators and gear to work the larger shells). The only people who ever increased gun caliber while retaining the turret were the Italians in the Conte di Cavour and Andrea Doria classes, which was possible because they used essentially the same guns with new liners and thinner barrels (the thick barrels of the WWI-era wire wound British guns allowed for that). German gun barrels were much thinner to begin with, so that would not have been possible either.
There however was a pre-design for Seydlitz mounting eight 305mm guns arranged as the 280mm guns on Von der Tann; beats me why this was not followed through, given that German battleships had switched to 305mm ordnance even before Seydlitz was laid down. Such a ship would have looked something like the AU Graf Schwerin to be found here: