If I have a buffed unit with Warmog's attached and I conquer Monastery of Hirana, can I use the buff to draw 1, and then re-buff from Warmog's?

Yes, you can do this by carefully ordering the triggered abilities on the chain.

According to Rule 702.3.a, a unit can only have one buff at a time. If you were to resolve the Warmog’s Armor trigger while your unit already had a buff, the new buff would not be placed. By resolving the Monastery of Hirana ability first, you spend your existing buff, which clears the "buff slot" and allows the Warmog's Armor trigger to successfully apply a new buff to your unit.

The Sequence

When you conquer the Monastery of Hirana with an equipped, buffed unit, both abilities trigger simultaneously. You choose the order in which they are placed on the chain:

1. Trigger Placement: Place the Warmog's Armor trigger on the chain first, then place the Monastery of Hirana trigger on top of it.
2. Resolution (LIFO): The chain resolves Last-In, First-Out (Rule 336.1). The Monastery of Hirana trigger resolves first.
3. Spend and Draw: You spend your existing buff to draw a card. Your unit now has no buff.
4. Gain New Buff: The chain continues to the next item, which is the Warmog's Armor trigger. Since your unit no longer has a buff, the condition is met, and you receive a new buff.

Summary of Key Rules

• Trigger Ordering: When multiple abilities trigger from the same event, you choose the order they are placed on the chain.
• Buff Limits: A unit can only have one buff at a time (Rule 702.3).
• Spending Buffs: Spending a buff is a cost paid during the resolution of an ability, not when the ability is placed on the chain.

This interaction is consistent with RiftJudge FAQ #9139.