To become a Dragon Disciple, you can't be a half-dragon. But a Dragon Disciple gains the half-dragon template at L10. Thus, a L10 Dragon Disciple does not qualify to become a Dragon Disciple. According to the CW rules, he loses access to all class features of the PrC, which includes the template. Since he's no longer a half-dragon, he now qualifies for the PrC. Repeat ad absurdium.