RE: Changing SD Card
The detailed data which has already been imported by Sleepyhead does indeed remain on your hard drive.
Your machine only stores basic data internally and this will be written to your new card when you insert it. The "high resolution" data such as the breath by breath flow rates will be written directly to the card but not stored in the machine. So the new card will have all the basic data from when you started using the machine, and the hi res data from today forward.
RE: Changing SD Card
If you manually copy the files from the card to a temporary storage space on your hard drive, then copy those onto the new card, you should have everything if you are just starting with the new card. If you're trying to combine data from 2 different cards, that may not work - Resmed seems to have a system for file names which increments in some way, so you may wind up with files having the same name on the card for the machine.
Note: I'm an epidemiologist, not a medical provider.