Conversion can be seen in Units > Awaken Units > Conversion (top right of the Awaken Units' screen). The conversion menu will show the items you can obtain by converting a certain unit before the conversion actually takes place. Once the action is done, the conversion will grant you that unit's [A] prism, and [B] a prism moogle with that unit's Trust Master Reward.

Notes:
• Only units with their 7★ awakenings unlocked may be converted.
• Once units are converted into prisms, they cannot be reverted to their original state.
• Prism moogles retain the same Trust Master value as that of the original unit the Trust Master belonged to before the unit's conversion.
• A conversion will not yield any prism moogles if the Trust Master Reward of the unit being converted has already been claimed.