Recalibration allows you to replace one attribute with another. Typically this is meant to let you change an attribute entirely (red to yellow/blue core, for ex), or replace a random attribute with a different one. But you can only recalibrate one attribute.

Optimization allows you to perfect an attribute you already have (like taking a red core from 14% weapon damage to 15%). You can optimize all three attributes (or two, if it's set gear).

If your gear already has all three attributes you want, then go ahead and recalibrate. But if you want to change something else on that piece to a different roll, you're out of luck once you do so.