I your situation close old account and open a new one?
I dont like rewriting history in my journal personally.
What I do personally is I got one Saving account in real life and manage some "virtual" account under that parent in beancount. And my balance assertion are always towards the real parent account.
* Assets:Bank:Saving
2017-01-01 open Income:Interest:Saving CAD
2017-01-01 open Assets:Bank:Saving CAD
2017-01-01 open Assets:Bank:Saving:Payments CAD
2017-01-01 open Assets:Bank::Saving:Projects CAD
2017-02-01 balance Assets:Bank:Alterna:Saving 0.00 CAD
Payments is money I save for yearly payments, like house taxes, car resgistration, insurance, license, etc..
And projects is money I save for short/mid term expenses.
In your case you could add sub account for projects if you want more detail of what the money was for then close once your are done.