Celestial Colonnade

Once Celestial Colonnade has attacked, tapping it for mana won't remove it from combat.

If Celestial Colonnade can't attack unless you pay a cost that includes a mana payment, you may attack with it and tap it to pay for that cost. It will still attack in this case.

When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.