| LuxuriantOak |
This is not RAW, but in my opinion taking the full levels (or maybe 7?) in Dragon Disciple IS pretty much becoming a half-dragon.
templates are iffy and increase the power level of characters outside the normal experience curve.
so, to Keep It Simple Stupid: just keep leveling the Dragon Disciple class, watch your charcater slowly embrace his draconic heritage and gain power until at it's peak he can turn into a dragon himself (although quite the small one in my opinion.)
on the questions on how the template and prestige class interact ..
I honestly don't know. A cursory glance tell me they give out some very similiar boosts and powers - althought the half-dragon seems more powerful than the DD.
I do not know if they stack. I would say no, but more rules-savy posters might contribute on the juridic side of things.
But Common Sense(tm) (a 3pp feat for all characters) says that a half dragon can't become MORE half-dragon-y. the DD has a goal to become more dragon-like, at his peak he is not really a dragon, but definitive more than just a man(woman, elf, dwarf, kobold, whatever). The half dragon has already achived all these things when the template gets added (usually at birth btw, or before if you want to get all biological about it)
I would say pick one: either level as a DD or, if your gm allows it, add the half-dragon template and retrain the DD levels.