changing an expense amount on a filed return

Yes, you can still make changes after you have filed your tax return. You can use ReFile in TurboTax or you can use the Canada Revenue Agency (CRA)’s My Account service to make the changes. But you need to wait until you have gotten your Notice of Assessment. This TurboTax FAQ has more information on changing a filed return: I've filed my tax return. Can I still make changes?

Does this mean when your refund is actually deposited, or when you see your NOA online, which I can do now

Yes now that the assessment is done for certain, you can edit your return or make a change.

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