Index: binaries/data/mods/public/simulation/ai/common-api/entity.js
===================================================================
--- binaries/data/mods/public/simulation/ai/common-api/entity.js
+++ binaries/data/mods/public/simulation/ai/common-api/entity.js
@@ -346,6 +346,13 @@
 		return templates.replace(/\{native\}/g, this.civ()).replace(/\{civ\}/g, civ).split(/\s+/);
 	},
 
+	"upgradableEntities": function(civ) {
+		const choices = this.get("Upgrade");
+		if (!choices)
+			return undefined;
+		return choices;
+	},
+
 	"researchableTechs": function(gameState, civ) {
 		const templates = this.get("Researcher/Technologies/_string");
 		if (!templates)
Index: binaries/data/mods/public/simulation/ai/petra/config.js
===================================================================
--- binaries/data/mods/public/simulation/ai/petra/config.js
+++ binaries/data/mods/public/simulation/ai/petra/config.js
@@ -36,6 +36,13 @@
 		"Fire": 0.095
 	};
 
+	this.DefensiveStructureDamageTypeImportance = {
+		"Hack": 0.075,
+		"Pierce": 0.095,
+		"Crush": 0.065,
+		"Fire": 0.085
+	};
+
 	this.Economy = {
 		"popPhase2": 38,	// How many units we want before aging to phase2.
 		"workPhase3": 65,	// How many workers we want before aging to phase3.
Index: binaries/data/mods/public/simulation/ai/petra/headquarters.js
===================================================================
--- binaries/data/mods/public/simulation/ai/petra/headquarters.js
+++ binaries/data/mods/public/simulation/ai/petra/headquarters.js
@@ -130,9 +130,14 @@
 	{
 		if (evt.to != PlayerID)
 			continue;
+
 		let ent = gameState.getEntityById(evt.entity);
 		if (!ent)
 			continue;
+
+		if (evt.from === PlayerID && ent.getMetadata(evt.from, "upgrading") !== undefined)
+			ent.setMetadata(evt.from, "upgrading", undefined);
+
 		if (!ent.hasClass("Unit"))
 		{
 			if (ent.decaying())
@@ -1569,6 +1574,43 @@
 		}
 	}
 
+	// Check for possible upgrades
+	if (!this.saveResource)
+	{
+		// TODO: Sort by most active defences, tracked via by how recently they last attacked an enemy.
+		const upgradableDefenses = gameState.getOwnEntitiesByClass("Defensive", true).filter(ent => ent.upgradableEntities() && !ent.getMetadata(PlayerID, "upgrading"));
+		for (const ent of upgradableDefenses.values())
+		{
+			const upgradableEnts = ent.upgradableEntities();
+			let bestAttack = PETRA.getMaxStrength(ent, this.Config.debug, this.Config.DefensiveStructureDamageTypeImportance);
+			let chosenUpgrade;
+
+			for (const choice in upgradableEnts)
+			{
+				const template = gameState.getTemplate(gameState.applyCiv(upgradableEnts[choice].Entity));
+				if (!template)
+					continue;
+
+				// Notice that AI entities are derived from templates and can be passed to most entityExtend functions.
+				const templateAttackStrength = PETRA.getMaxStrength(template, this.Config.debug, this.Config.DefensiveStructureDamageTypeImportance);
+
+				// Dont want a weaker upgrade here since this concerns defense.
+				if (templateAttackStrength < bestAttack)
+					continue;
+
+				// If the upgraded entity's strength is the same but has less health, we don't want it.
+				if (templateAttackStrength == bestAttack && template.maxHitpoints() < ent.maxHitpoints())
+					continue;
+
+				bestAttack = templateAttackStrength;
+				chosenUpgrade = upgradableEnts[choice];
+			}
+
+			if (chosenUpgrade)
+				queues.defenseBuilding.addPlan(new PETRA.UpgradePlan(gameState, ent.id(), chosenUpgrade));
+		}
+	}
+
 	if (this.Config.Military.numSentryTowers && this.currentPhase < 2 && this.canBuild(gameState, "structures/{civ}/sentry_tower"))
 	{
 		// Count all towers + wall towers.
Index: binaries/data/mods/public/simulation/ai/petra/queue.js
===================================================================
--- binaries/data/mods/public/simulation/ai/petra/queue.js
+++ binaries/data/mods/public/simulation/ai/petra/queue.js
@@ -149,6 +149,8 @@
 			plan = new PETRA.ConstructionPlan(gameState, dataPlan.type);
 		else if (dataPlan.category == "technology")
 			plan = new PETRA.ResearchPlan(gameState, dataPlan.type);
+		else if (dataPlan.category == "upgrading")
+			plan = new PETRA.UpgradePlan(gameState, dataPlan.entityID, dataPlan.upgradeData);
 		else
 		{
 			API3.warn("Petra deserialization error: plan unknown " + uneval(dataPlan));
Index: binaries/data/mods/public/simulation/ai/petra/queueplanUpgrading.js
===================================================================
--- /dev/null
+++ binaries/data/mods/public/simulation/ai/petra/queueplanUpgrading.js
@@ -0,0 +1,72 @@
+PETRA.UpgradePlan = function(gameState, entityID, upgradeData)
+{
+	const ent = gameState.getEntityById(entityID);
+	if (!ent || ent.upgradableEntities().every(upgrade => upgradeData.Entity !== upgrade.Entity))
+		return false;
+	ent.setMetadata(PlayerID, "upgrading", true);
+
+	this.entityID = entityID;
+
+	this.upgradeData = upgradeData;
+	this.cost = new API3.Resources(upgradeData.Cost);
+
+	// We do not call the base constructor, so set the ID here.
+	this.ID = gameState.ai.uniqueIDs.plans++;
+	this.category = "upgrading";
+	this.type = "upgrading";
+	this.number = 1;
+
+	return true;
+};
+
+PETRA.UpgradePlan.prototype = Object.create(PETRA.QueuePlan.prototype);
+
+PETRA.UpgradePlan.prototype.canStart = function(gameState)
+{
+	const requiredTech = this.upgradeData.RequiredTechnology;
+	return !requiredTech || gameState.isResearched(requiredTech);
+};
+
+PETRA.UpgradePlan.prototype.isInvalid = function(gameState)
+{
+	const ent = gameState.getEntityById(this.entityID);
+	return !ent || !ent.getMetadata(PlayerID, "upgrading");
+};
+
+PETRA.UpgradePlan.prototype.start = function(gameState)
+{
+	Engine.PostCommand(PlayerID, {
+		"type": "upgrade",
+		"entities": [this.entityID],
+		"template": gameState.applyCiv(this.upgradeData.Entity),
+		"queued": false
+	});
+
+	this.onStart(gameState);
+};
+
+PETRA.UpgradePlan.prototype.onStart = function(gameState)
+{
+	if (this.queueToReset)
+		gameState.ai.queueManager.changePriority(this.queueToReset, gameState.ai.Config.priorities[this.queueToReset]);
+};
+
+PETRA.UpgradePlan.prototype.Serialize = function()
+{
+	return {
+		"entityID": this.entityID,
+		"upgradeData": this.upgradeData,
+		"category": this.category,
+		"cost": this.cost.Serialize(),
+		"queueToReset": this.queueToReset || undefined
+	};
+};
+
+PETRA.UpgradePlan.prototype.Deserialize = function(gameState, data)
+{
+	for (const key in data)
+		this[key] = data[key];
+
+	this.cost = new API3.Resources();
+	this.cost.Deserialize(data.cost);
+};