How the packs work is medium has 6 themes and they are all preselected, so no swaping. Adding HD onto the medium combo is the same price as the large combo.
You can go ahead and select 6 themes of your choice and add in HD, but it's going to be more expensive to do it that way than the your pick HD.
6 themes x7 42
HD (3 or more) 15
Large has 9 themes. Although on the website they list all 9 themes, in actuality they are the same preset themes as the 6 medium ( adventure, entertainment extra, news, lifestyle and family ) and the remaining 3 (entertainment, sports and timechoices ) are interchangeable with any of the other themes. It makes no sense to add on HD to this pack. You're better off in your pick HD
The your pick gives 9 themes as well as the HD version of the themes you choose.
People go with the large if they want no HD but 1 or 2 more themes than the medium because it's a better value than just having an extra couple themes al a carte.
My opinions and time are all my own.