if you read my post containing the save, you'd know you're supposed to import the ID from your old save - or at least generate one with a new save to copy over. Half of the ID seems to be tied to your system (the first 4 bytes are the same for all 3 saves if made on the same 3DS), any non-zero number will work (zero technically works but it makes your GC both unblockable and unviewable - thus it's nether recommended nor useful) but you run the risk of someone else having/picking that number, thus having the two register as the same person's Guild Card (side effects are you replace each other's cards via streetpass/sharing in lobbies, you're unable to send your GC to/receive a GC from with the other person, and blocking GCs from either one of you blocks both). Copying from one of your own saves (existing or newly made) simply lets the RNG and system ID do the work of ensuring a unique number.
The name on the Guild Card will update after you save in-game at least once.