I got to download the file! 

REALLY awaiting Marks answer.

That C5 file is grainy in any of the screen "Size" versions renders in C6. I swapped the sky with a new realistic in C6--same result--grainy?
It's a big worry I think that all those, many, C5 files (OF MINE!) may  also not be able to be properly worked on in the future in C6?
What else may be effected in all my many C5(and older) file saves?

bwtr