Here's a
characters/seiyuus page to help keep the characters and their seiyuus straght. Pics of characters and their seiyuus, with info about each seiyuu.
This is from a novel, so I suspect the author has worked out an interesting reason for the switches, and has a fuller background for Heartseed. At least I hope so.
As for the word "heartseed," we should not take its English associations too literally. In Japanese, the word used is
fuusenkazura 風船葛, or "balloon-vine." It's an annual vine that seems to grow quickly.
Pics. Why he chooses that name I really have no idea.