Can you 'upgrade' specific magic item property runes?

Question

Specific Magic items have the limitation

You can't etch or transfer any property runes onto a specific weapon that it doesn't already have, and you can't remove its property runes.

You can't etch or transfer any property runes onto a specific armor that it doesn't already have or remove its property runes.

However, I'm curious how this works when the runes have Greater (or possibly higher) versions. A couple examples would be

• Could you upgrade a Storm Flash's Shock rune to Shock (Greater)?
• Could you upgrade an Energizing Lattice's Fortification to Fortification (Greater)?

The crux is, of course, whether making a rune Greater (etc.) constitutes applying a rune "it doesn't already have".

Answer 1

Depends on the rune (but yes)

The rules for Specific Magic Weapons state that:

These weapons have abilities far different from what can be gained by simply etching runes. A specific magic weapon lists its fundamental runes, which you can upgrade, add, or transfer as normal. You can't etch or transfer any property runes onto a specific weapon that it doesn't already have, and you can't remove its property runes.

Note the "doesn't already have" part, which implies by omission that you can etch or transfer a rune that the specific weapon already has.

To understand this we need to look at rules for Reading Items, specifically those about Multiple Types and Crafting Requirements:

If multiple types of an item exist, the title line gives the minimum level followed by a plus symbol (“+”). The description includes information on the base version of the item, and the Type entries at the bottom of the stat block lists the specifics for each version, including the level, Price, and any modified or added abilities of the different types. For some items, the types listed are upgrades to the base item. For other items, such as aeon stones and wondrous figurines, each type is distinct from the others.

The GM might allow you to Craft a permanent item from a lower-level version of the same item as an upgrade. For example, you might upgrade a bag of holding from a type I to a type II bag, but you couldn't upgrade a clear spindle aeon stone into an orange prism aeon stone. The cost for this upgrade is the full difference in Price between the items, and the Crafting check uses a DC for the item's new level.

Currently, there are no property runes with types that are distinct¹, but if such a rune were to exist on a specific weapon, you would not be able to upgrade it.

On the other hand, there are property runes with types that are upgrades, and those you can, well, upgrade.
For example, Shock has two types: a base version, and a greater version that is clearly an upgrade of the former. You could etch either type of Shock onto a specific weapon like Storm Flash, because it already has a Shock rune. If you etch the greater version, you can upgrade the existing Shock, as described above and in the rules for Runes:

An item can have only one fundamental rune of each type, though etching a stronger rune can upgrade an existing rune to the more powerful version (as described in each rune’s entry).

Although that is specifically about fundamental runes, it applies to property runes too, as a later paragraph states that:

You can upgrade a property rune to a higher-level type of that rune in the same way you would upgrade a fundamental rune.

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1. Even a rune like Bane, which only works on one creature type, still doesn't have the entries for multiple types.

Answer 2

You Can Upgrade Property Runes on Specific Magic Weapons

The rules on runes mention that property runes are upgradable:

You can upgrade a property rune to a higher-level type of that rune in the same way you would upgrade a fundamental rune.

You'll notice that they also mention that upgrading is done by etching:

...though etching a stronger rune can upgrade an existing rune to the more powerful version.

Which is the crux of the dilemma, as the specific magic weapon rules state:

You can't etch or transfer any property runes onto a specific weapon that it doesn't already have.

So, in order to upgrade a property rune, we must determine whether upgrading to a Greater version of a rune counts as etching a rune that "it doesn't already have" (as you point out in your question).

So does it?

You Can Have Different Levels of the Same Rune

The property rune rules include this line:

If a suit of armor or a weapon has multiple etchings of the same rune, only the highest-level one applies.

If "same rune" was meant to be the same version of a rune, then they'd always be the same level, so there wouldn't be a need for this clause at all. This makes it pretty clear that we can expect to have "same runes" of different levels, thus implying that Flaming is the "same rune" as Flaming (Greater).

You Can Etch Runes The Weapon Already Has

The specific magic weapon rule in question has this interesting caveat:

You can't etch or transfer any property runes onto a specific weapon that it doesn't already have, and you can't remove its property runes.

This is a very strange clause to have if you can't upgrade your property runes on your weapon. They could simply say "you can't add or remove property runes to a specific magic item." But the rules carve out a specific exception for runes a specific magic weapon already has - which makes no sense unless you can upgrade. Why would you want to add Shocking to Storm Flash? It has Shocking already, and the duplicate rune rules say you would then get no benefit from it! So there would be no reason to include this clause at all. So clearly they intend to have the runes on it upgradable.

Conclusion

Given that the rules expect you to etch runes onto a specific magic weapon, and that they treat different leveled version of the same rune as the same rune, it is clear that you can upgrade property runes on specific magic weapons and armor.

Answer 3

You Can Upgrade Property Runes

The full rules for property runes includes the following text, emphasis mine:

Property runes add special abilities to armor or a weapon in addition to the item’s fundamental runes. If a suit of armor or a weapon has multiple etchings of the same rune, only the highest-level one applies. You can upgrade a property rune to a higher-level type of that rune in the same way you would upgrade a fundamental rune.

For reference, the full rules on fundamental runes, including upgrades, are listed here.