Or, depending on how the code is written, calculate the new modifier for the creature's position, then overwrite the old with the new.
Move Action
When a creature moves from one zone to another, there is a specific sequence of events which must be followed. A creature might move to a new zone by taking a move action, or it might be Pushed or Teleported. Regardless as to how it moves to the new zone, it follows the move sequence below, although some parts or steps might be irrelevant. For example, a Teleport bypasses Walls and ignores Step 3 below.
1. Declare Move
Choose which zone the creature will move to. Check to see if the move is possible (for example, make sure no wall will block the move), and make sure the creature has an available action and is not Restrained, Rooted or Incapacitated. If you discover you made a mistake and the creature cannot be moved (for example, the wall is “Passage Blocks”, not “Passage Attacks”), you can cancel the move action and choose a different action instead. Then, pay any costs associated with the move (taking damage, or paying Suppression Orb's mana cost, etc.). At this time the move is committed to. If you cannot pay all of the costs, the move is cancelled, and you have lost the action.
2. Leaving Zone Effects
Some spells or abilities might have an effect which triggers when a creature is leaving a zone. Note that there currently are no “Leaving Zone” effects in the game, but future ones will be added.
3. Walls
The creature encounters any intervening walls (unless it is Teleporting, in which case it ignores walls).
If a creature has been Pushed into a wall with the Passage Blocks trait, it becomes Bashed and receives a Bash Attack now. If the creature is moving through a wall with the Passage Attacks trait, it conducts that attack at this time.
4. Move to New Zone
The creature must be moved into the new zone. It is possible that a wall attack might Restrain or Incapacitate a creature, in which case it will not be able to complete its move action, and will not be able to move into the new zone. See “Walls” below.
5. Entering Zone Triggers
Any spells, abilities, or effects which trigger for a creature entering a zone occur at this time. Examples include a trap or Mangler Caltrops spell. You do not get a chance to reveal enchantments after each of the 4 events above, like you do with the attack and casting sequences. (However, if there is a wall attack, you can reveal enchantments after each Step of the attack as normal. See “Walls” below.) If a creature becomes unable to move during the move action, it stops the move sequence at that point, and the rest of the move action is canceled. For example, a creature could become Restrained from a “Leaving Zone Effect”. If this happens, the rest of the move action is canceled, and it will not encounter any walls, nor actually be moved into the new zone.
The zone change happens in step 4. The Armor is not a caused by a trigger, it's a conditional trait. If, at any time, the creature satisfies the condition ("in this zone") it has the Armor +2 trait.
Now, the truth is that I'm not completely sure that this is how it works, but I'm very sure we don't want to start resolving conditional traits as actions, because it'll break other stuff. Conditional traits can't happen, they just have to be a consequence of something else that happens.