What happens when a spellcaster with Legendary Resistances Shapechanges into another creature with Legendary Resistances?

Question

A few high level arcane spellcasting monsters, such as the Lich from the Monster Manual, also have Legendary Resistances (here, 3 per day).

Such spellcasters can prepare then cast Shapechange on themselves to become a legendary creature (such as an Ancient Brass Dragon), while keeping some things:

You retain the benefit of any features from your class, race or other source and can use them, provided that your new form is physically capable of doing so.

It is clear from the Monster Manual's errata that there are several things not gained from assuming the form of another creature:

If a creature assumes the form of a legendary creature, such as through a spell, it doesn't gain that form's legendary actions, lair actions, or regional effects.

But there is no mention of Legendary Resistances here.

What happens when a spellcaster with Legendary Resistances casts Shapechange on itself to become another creature that also happens to have Legendary Resistances?
(For the example's purpose, let's say that the Lich has used 1 of its 3 resistances before Shapechanging into the Dragon.)

1. The caster's legendary resistances (here, 2) get overridden by the resulting creature's legendary resistances (here, 3) but become available again when the spell ends
2. Both sets of legendary resistances are available at the same time (for a total of 5)
3. Some other scenario

Answer 1

As per the PHB description for shapechange:

You retain the benefit of any features from your class, race, or other source and can use them, provided that your new form is physically capable of doing so.

I’d lump legendary resistances under such features. However, your statistics are replaced, including presumably the number of times you can use your legendary resistances:

Your game statistics are replaced by the statistics of the chosen creature

On this basis, it seems that the number of legendary resistances you would normally have would be overwritten by the number the new form has when you are shapes changed into a creature with legendary resistances.

Going the other way is not so clear, since there is no mention of what happens to such features when you revert. The closest analogue would be HP, as a sort of consumable. On this basis, you could argue that while you are in a particular form, you have access to exactly the number of legendary resistances as is specified in the stat block, and that these figures do not stack. Instead, when you revert you have remaining the number you had before you shapechanged, i.e. scenario 1.

To clarify, legendary resistances are not legendary actions, and therefore are not excluded as part of the spell, according to an unofficial tweet by rules designer Jeremy Crawford:

The spell shapechange states that the you can't use legendary actions. But, does legendary resistance counts as a LA?

Legendary actions appear in a stat block under the heading "Legendary Actions."