I'm curious as to whether a Feat like this would be too powerful.
Benefit: You are able to choose between Positive or Negative energy whenever you use Channel Energy.
If you normally channel Positive energy, you may expend 2 uses to change the energy type to Negative.
If you normally channel Negative energy, you may expend 2 uses to change the energy type to Positive.
Having this feat means you qualify for feats and abilities that have "channel positive energy" or "channel negative energy" as a prerequisite (for example, you qualify for the Command Undead feat and the Turn Undead feat). However, you must expend 2 uses of Channel Energy to use your opposite energy type in conjunction with those feats.
It's similar to the Versatile Channeler feat, but better and worse at the same time. This has less restrictions, but you can only use Channel Energy half your normal amount of times for the opposite energy type. Versatile Channeler simply reduces your Cleric level by 2 for the purposes of channeling your opposing energy type, and that's all. In the end, you get more uses out of Versatile Channeler, but Opposite Channels have a more powerful effect (but only by a single die) and is easier to qualify for.
I'm looking to balance this feat as much as possible, as I'm thinking of giving it to an NPC that worships an evil god, but is not, herself, evil.
Prerequisites: Channel energy class feature, necromancer or neutrally aligned cleric (see below).
Benefit: You may make a choice whenever you use your channel energy class feature.
If you normally channel positive energy, you may choose to channel negative energy as if your effective cleric level were 2 levels lower than normal.
If you normally channel negative energy, you may choose to channel positive energy as if your effective cleric level were 2 levels lower than normal.
Having this feat means you qualify for feats and abilities that have "channel positive energy" or "channel negative energy" as a prerequisite (for example, you qualify for the Command Undead feat and the Turn Undead feat).
Note: This feat only applies to necromancers, neutral clerics who worship neutral deities, or neutral clerics who do not worship a deity -- characters who have the channel energy class ability and have to make a choice to channel positive or negative energy at 1st level. Clerics whose alignment or deity makes this choice for them cannot select this feat.
[Edit] Another possibility is to reduce your effective Cleric level by half, for the purpose of channeling your opposite energy type.
Maybe even a combination or reduction of Cleric level and additional cost would work? Like a -2 level reduction, and costing 2 uses of Channel Energy.