lmao, this is crazy, I always thought the memory is soldered onboard or something for devices with large memory capacities.
Of course not ! The HD7 is nothing else but an HD2 missing a button and with a hidden but ordinary memory card slot.
This had been known from the beginning. This tutorial explains perfectly how to do it, thanks to Wolfman017... But it has been told so many times that everybody on the Earth should have know the fact !
Better keep your HD2 and flash WP7... Provided you like an OS unable to do anything of course, but, well, for the fun...
If I ever run out of my "16GB" of space, ofcourse I'll probably be doing this.
If you ever run out of your 16 GB space, it will be quite late, as swapping will of course leave your data on the memory card that has been removed... Plus it requires a hard reset, so anything present on the new card will be wiped out.
So do it now, before putting anything on it... Or never do it.
Or find a way to efficiently backup your data, and patiently reinstall all you had on your phone.
Furtunately, I happen to precisely have a 32 GB Sandisk Class 2 memory card in my HD now...
I know it will always be useful, starting by using it on a potential HD2, with or without WP7.