I have never used the 8K, though my thoughts are as long as it removes metal and is a finer grit you can use it in your next step. Now whether or not you spend the next year removing those scratches is another story.

Based on what Bart said it seems going from the 1.2K to the 8K shouldn't be a problem. I go from the 1.2K to my BBW all day with no problems, it's not quite the same but still.