Just to further clarify and enhance shark's explanation:
The rules for revealing enchantments are that you may reveal an enchant immediately after any "step" in a sequence. The current upkeep phase has zero steps in it. It is illegal to reveal any enchantment during upkeep. However, the steps to a game round are:
Initiative, Channel, Upkeep, Planning, Deployment, etc. Therefore there is a step (channeling) after which you have mana and can reveal enchantments that will affect upkeep.
My proposed upkeep change has no effect on this mechanic. All it does is streamline interactions during upkeep while keeping the enchantment reveal problem out of the phase.
Honestly, Pillar of Righteous Flame should do its attack at the end of the round because it adds steps to a phase that shouldn't have them.