When I change my account balance in the "edit account" section it changes my unallocated balance. I messed up somewhere and need to change my unallocated balance...but when I do, it changes my account balance.
I believe I just responded to your email, so hopefully we can help you get your Unallocated issue worked out together there.
But to answer your direct question -- it is not possible to change the balance of one of your Accounts without it affecting your Unallocated. This is because Unallocated, as the balance that shows how much money you have available to budget across all of your Accounts, is directly connected to your Accounts' balances.