I have a Dwarf who uses a Dorn-Durger.
He just picked up Dorn-Durger Master to be able to use it in one hand.
Dorn Durger Master has a prerequisite of Two-Weapon Fighting, which I can't really see this character ever using.
If I were to retrain two-weapon fighting to another feat, would I lose the ability to use Dorn-Durger Master?
You can't do it.
Feat
You may change one feat to another through retraining. Retraining a feat takes 5 days with a character who has the feat you want. The old feat can't be one you used as a prerequisite for a feat, class feature, archetype, prestige class, or other ability. If the old feat is a bonus feat granted by a class feature, you must replace it with a feat that you could choose using that class feature.
Note that this retraining is unrelated to the fighter ability to learn a new bonus feat in place of an old one at certain class levels. That class ability is free, happens instantly when the character gains an appropriate fighter level, doesn't require a trainer, and can happen only once for any appropriate fighter level. Retraining a feat requires you to spend gp, takes time, requires a trainer, and can happen as often as you want.
Also, if a dm were to allow you to retrain it anyway...
Prerequisites
Some feats have prerequisites. Your character must have the indicated ability score, class feature, feat, skill, base attack bonus, or other quality designated in order to select or use that feat. A character can gain a feat at the same level at which he gains the prerequisite.
A character can't use a feat if he loses a prerequisite, but he does not lose the feat itself. If, at a later time, he regains the lost prerequisite, he immediately regains full use of the feat that prerequisite enables.
That is really unfortunate. I guess I just get to light a feat on fire to make my concept work.
It is a bit strange as a prerequisite since the dorn dergar is not a double-weapon, but there it is. I suppose you could use two of them. : D
I think that's the idea. I'd prefer that be an option for the fighting style rather than the expectation.
The same was done with the dervish dancer, you need weap finesse to be able to apply your dex mod to attack and damage rolls, even though the scimitar is not a finessable weapon.