Darting Viper (Feat)

Prerequisites: BAB +4, proficiency with an inclusive-reach weapon

Benefit: Whenever you change your inclusive-reach weapon between a normal or reach weapon, your next attack gains a +2 circumstance bonus to your attack roll.

Note: When fighting size large and larger creatures, no, you can't choose to attack a further-away square of the monster's space to gain this bonus. (sigh)