Stats of already possesed items changed in patch

Solution 1:

The 2.4.0 PTR patch notes currently indicate that changes to legendaries, both class-specific and non-class-specific, will not affect existing items except where otherwise specifically noted (and there are a few cases, such as The Tall Man's Finger, where it is).

Several existing Legendary items have been updated to include a unique Legendary power Note: Existing items will not be affected by the following changes. Only new versions of the items will roll with the added Legendary power.

Although things could well change between the PTR and the final release, The Fist of Az'Turrasq is not presently labelled as being a retroactive change.

The retroactive changes that are marked in 2.4.0 PTR appear to all be changes that don't involve changing the affix description. For example, The Tall Man's Finger describes a change to your Zombie Dogs, but the affix never describes how much health the new ZD has, except that it's more than your old dogs had combined. A change to this health bonus doesn't require any change to the affix description. I'm speculating that the change is retroactive because no change to the affix description is required. But don't take that as gospel.

I also wouldn't expect reforging an old item in Kanai's Cube to update it to the new item affixes. When we use the term rolling, we're talking about rolling the values inside each affix. Those values can fall within a limited range. The affixes themselves don't change. It's probably best to think of the old item and the new one as completely different items that happen to share a name and appearance. (EDIT: I'm getting a lot of people tell me that they think this isn't the case, but nobody seems to have tested it.)

I'm going to start a new question around what the consequences of this might be for set bonuses.

EDIT: an answerer to my question on sets has indicated that set items are typically updated retroactively to prevent this situation.