FYI...my H also can NEVER come up with a sexual fantasy. The interesting thing is that when I finally told him...your fantasy doesn't have to be something off the wall, it could be something that most people would consider perfectly normal...but you haven't tried it, or maybe it's something you have tried...but you really enjoy. He NEVER even considered those options when I would ask him to tell me a fantasy. When I asked him if he thought I wanted him to come up with some sexual situation that was off he wall...he said "well yeah." THAT was why he couldn't come up with ANYTHING at all. His mind was so busy trying to conjur something wild up that it came up with nothing at all.
And yes people can switch from LD to HD...I and other people on here have.