Okay, these desperately need new stickers, but that aside, here is the quick and dirty version of my "Cube in Sphere" and "Sphere in Cube". (That's my finger on the bottom left. Nice, isn't it?)
I popped the caps on three of the center pieces on each puzzle, switched them, then reassembled. Took about thirty minutes including shameful sticker moving. These puzzles were both unsolvable when I got them. Someone had fun moving stickers around. Two green stickers on one edge piece and other fun things. As you can see, many of them are missing - a sure sign that they were moved around a little too much! You'd think if someone was going to take the time and trouble to move stickers, they'd at least put them in the right spot!)
Maybe this partially answers Doug's question about what people are doing with all the identical cubes they buy on eBay. I have bought a number of identical items, some in pretty rough shape (such as these two). When I can get a good deal on a visually banged up item that I know I'm going to modify and resticker anyhow, I jump on it. I'm still buying up identical items of certain shapes to fit other modification plans on the horizon.