If I have a buff and Warmog's attached, I conquer Monastery of Hirana. Spend a buff, draw a card, and then get a buff.

To end with a buff after conquering the Monastery of Hirana while already possessing a buff and having Warmog's Armor attached, you must carefully order the resolution of the triggered abilities.

The Ruling

Yes, you can spend your existing buff to draw a card and then gain a new buff from the Warmog's Armor trigger, provided you order the triggers correctly on the chain.

According to Rule 702.3.a, if you attempt to add a buff to a unit that already has one, the new buff is not placed. Therefore, if you resolve the Warmog's Armor trigger first, it will fail to provide a new buff because your unit is still holding the original one. You must resolve the Monastery of Hirana's ability first to spend the existing buff, clearing the slot for the new one.

The Sequence

When you conquer the Monastery of Hirana, both the Monastery's ability and the Warmog's Armor ability trigger simultaneously. As the controller of both, 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): Because the chain resolves Last-In, First-Out (Rule 336.1), the Monastery's ability resolves first.
3. Spend and Draw: During the resolution of the Monastery's ability, 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). If you attempt to add a buff to a unit that already has one, the action fails (Rule 702.3.a).
• Spending Buffs: Spending a buff is a cost paid during the resolution of an ability, not when the ability is placed on the chain (FAQ #2980).

By ordering the Monastery's ability to resolve first, you successfully clear the buff slot, allowing the Warmog's Armor trigger to resolve afterward and grant you a fresh buff.

Note: I could not fully verify the citation for trigger ordering against the retrieved rules.