The difference is negligible. "Therefore" means something like "for this (reason)" while "thereby" means "by this reason."
"I think therefore I am." gives "I think, and for this reason, I am." which holds a meaning of existing being true because thinking is true.
"I think, thereby, I am." gives "I think, and by doing so, I am." which means the slightly different existing being true as long thinking being true. The result is so subtle that you can easily use one for the other.