Item changes across patching?

I do not understand the system underlying how item changes become implemented across patches. There seem to be two very different and mutually exclusive methods for implementing change to items:
1. Pre-patch versions of that item retain their stats, only new item drops have new stats
2. Pre-patch versions of that item gain new stats and become identical to current drops

So some items (such as Kaom's Heart) are treated according to type 1, while others (e.g. Call of the Brotherhood) according to type 2.

What is the rationale for implementing two different types of item patching? Is there some way to determine which items will be treated according to type 1 or type 2?

This is due to how item data is stored, and predicting whether changes will be retroactive depends on the type of changes being made. An item's list of mods and their values is stored separately for each item. That makes changing them retroactively pretty much impossible, since it would take literal days of downtime for a script to run through, find and change every single instance of the item. However, changes to how a particular mod or mechanic works are necessarily retroactive, since that information isn't stored with the item - it's central and referenced when needed.

There's a bit of fudge factor here - GGG seems to prefer not creating legacy items where possible, so in some cases they change a mod's entire interpretation of its own values. For example, the life leech nerfs of 2.0 were retroactive because every life leech mod now just provides leech equal to 20% of the value listed for the mod - no need to update the mod values themselves. But that necessarily impacts every single item with the same mod, and usually creates unacceptable collateral, so it happens very rarely.

Basically, adding or removing mods cannot be retroactive. Changing the values of mods cannot either, but can be fudged if collateral damage isn't a problem. Updating the mechanics of a mod has to be retroactive.