Can I re-equip an equipped artifact to the same creature?
As our other Can you re-equip equipment to the same creature? question says, yes you can equip an equipment to a creature it is already equipped to.

However with Sunforger part of the cost of the ability is "Unattach Sunforger", this means that Sunforger will no longer be equipped to the creature it was equipped to prior to the ability being activated. This means that you are in fact not equipping the Sunforger to the creature that it is equipped to because the Sunforger is not equipped to any creature.