I don't believe there is a ledger on Item Master or Item Branch. I assume you'd know if your company set up some sort of custom ledger based on database trigger or something.
A couple things you might try. I might know of these because I've had to use one or the other over the years
1) Database journal if you're AS400
2) Historical data backup or test environment copied down from prod
3) If you trust your cardex F4111 UM you might try looking at cardex UM's for the same items before the change.
Probably easiest in the end to have someone with knowledge of the items to figure out the standard again and just manually fix each.