What's pinching? Are you feeling more pressure on the top of the index finger or the bottom of the ring finger? Since you're actually moving your hand up on the string going from three under to split finger, I'll assume it's the top finger, because the string angle on your ring finger would be less with split finger. It may be a matter of just toughening up the finger a little through shooting. It's just not used to feeling the string pressure there.
Most folks who shoot three under anchor much higher than you do/did -- high up on the side of their cheek and do what is called barrel sighting, i.e., looking down the arrow. This works very well for close range shooting, but not very well for farther range shooting. In this instance, moving to split finger usually involves the concommitant move to anchoring lower on the face near the corner of one's mouth. This has the effect of lowering the "rear site" and contributing to longer range accuracy.
However, since you already held pretty low with your three under hold, there's probably not much to be gained by going to split fingers.
If your finger keeps hurting, and you don't see any improvement in your accuracy or shooting comfort, I agree with the others. Don't change.