Hopefully, you’ve already figured it out, but just in case (and for others who may stumble on this) here is what I think would work.
In the Savings Budget sheet you would use the dropdown in the upper left to change it from adjusting Budget to adjusting Savings and then you can type all your existing savings amounts into the Adjust cell and have the other columns reflect your changes.
The last step would be to have it actually persist those adjustments by using the Labs add-on
You might also consider using the Savings and Debt sheet depending on whether you’re saving up for goals or just wanting to accrue savings for unspent category funds.