magic: the gathering enchantment problem
Bestow is a choice that you make while casting a spell. If you choose to cast it with the Bestow cost, it is an Aura spell instead of a Creature spell. This is a choice that you can only make while casting the spell. If the card is already on the battlefield, you can't turn it into an Aura with the Bestow ability.

You can enchant an Enchantment Creature with an Aura that has Enchant Creature, including an Aura that is a bestowed Creature Enchantment card. An Enchantment Creature is just both an Enchantment and a Creature, so it can be affected by things that affect Enchantments, and by things that affect Creatures, including Auras with Enchant Creature. Adding the Enchantment type to a Creature doesn't otherwise do anything special.