How to refer to a new member of an upgraded Struct

If I upgrade a struct within an upgradable contract (as described here,) that's being contained in a mapping, will the contract handle it gracefully if I refer to the new member on an existing mapping?

For example, if I had this contract:

contract LoyaltyV1 {
    struct Account {
        uint balance;

    mapping (address => Account) accounts;

Let's say I get a few accounts, then I upgrade the Account struct by adding a new member uint points. What happens if I call accounts[0].points (where accounts[0] was added before the upgrade)?

Will it return something like 0x0 or will it crash?

This will definitely break -- you can safely refer to points for a new Account instance, but old ones will have storage conflicts or errors.