Is there any lore explanation in Forgotten Realms for the change from Infravision to Darkvision?

Question

A long time ago, some races and monsters had the Infravision trait, which allowed them to see in the dark the heat traces of living creatures.

This had very interesting aspects. The drow's sign language\$^1\$ used the heat traces left in the air by the hands to convey messages. In order to become totally invisible in the darkness, one had to magically hide their body heat (see the attack to House DeVir in the novel Homeland).

Nowadays, since the 3rd edition has been published, the Infravision has been changed into the more manageable Darkvision, that allows to see in shades of gray instead of seeing the heat traces when in darkness.

Is there any in-world explanation in the novels, manuals, adventures or in other official sources about how this change occurred in the Forgotten Realms?

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\$^1\$ It has been adapted to the 3rd edition Darkvision trait, as explained in the Forgotten Realms Campaign Setting 3rd edition, see here.

Answer 1

Vecna blew up the multiverse.

Simply put, the in-universe explanation of the changes between 2e ADnD and 3e DnD was explained by the adventure Die Vecna Die! by Bruce R. Cordell and Steve Miller.

To quote the adventure itself:

Even with Vecna's removal, his time in the crux effected change in superspace. Though the Lady of Pain attempts to heal the damage, the turmoil spawned by Vecna's time in Sigil cannot be entirely erased. Some Outer Planes drift off and are forever lost, others collide and merge, while at least one Inner Plane runs "aground" on a distant world of the Prime. Moreover, the very nature of the Prime Material Plane itself is altered. Half-worlds like those attached to Tovag Baragu multiply a millionfold, taking on parallel realism in what was before a unified Prime Material Plane. The concept of alternate dimensions rears its metaphorical head, but doesn't yet solidify, and perhaps it never will. New realms, both near and far, are revealed and realms never previously imagined make themselves known. Entities long thought lost emerge once more, while other creatures, both great and small, are inexplicably eradicated. Some common spells begin to work differently. The changes do not occur immediately, but instead are revealed during the subsequent months. However, one thing remains clear: Nothing will ever be the same again.

The change between infravision and darkvision was part of the 2e to 3e rules updates; it would have been covered under this. In particular, it notes that "some common spells begin to work differently", and this would include the 2e spell Infravision and its 3e counterpart Darkvision.