In the attached pic, the one on the left is in my underwater GH4 until it's full, then I put the older one in the middle in. The one on the right is in my topside GH4. Had no problems with any of them.
Yes, I'd really miss that extra resolution now, for cropping/zooming/stabilization. Actually I think 6k would be great for cropping to 4k. I didn't crop any of these right down to 1080p, which starts to look not so nice. They were all scaled to some degree after the crop.