The changeling property in the Magic Item Compendium p.31 names a flat price of 2000 gp. It does not say that it counts or costs as much as a +1 enhancement.
Does that mean that the property doesn't count as an enhancement (and thus does not increase the price of the next enhancement and doesn't count towards the limit)?
Or does it count as a +1 enhancement but always cost a flat 2000 gp?
The magic weapon special ability changeling (Magic Item Compendium 31) (2,000 gp; 0 lbs.)—and other magic weapon special abilities with a flat cost—don't count as magical enhancement bonuses and aren't equivalent to magical enhancement bonuses. Such flat-cost magic weapon special abilities don't typically impact the cost of any of the weapon's other magic weapon special abilities, whether those other magic weapon special abilities exist upon the weapon now or are added later.
To be clear, nothing says that such flat-cost abilities do count as magical enhancement bonuses—and no way is given to determine the cost of a magic weapon were such abilities to count as magical enhancement bonuses—, so, by default, such flat-cost magic weapon special abilities don't count as magical enhancement bonuses. (In an exception-based game like 3.5, an exception for such abilities would have to be made clear for it to apply.)
However, a weapon must still possess at least a +1 magical enhancement bonus before such flat-cost magic weapon special abilities can be added to it. Further, those flat-cost magic weapon special abilities count toward whether or not a weapon is considered epic (see Magic Item Compendium on Weapon Properties (28)).
