Can I switch from paying by check to paying online if I've already filed and noted that I would pay via check?

[Karca]

You can't do it through TurboTax anymore once you have filed.  If you mean your federal taxes, you can go on the IRS.gov website and select "Make a payment" and pay by "Bank Account (Direct Pay)" before the end of the day.  If you mean your state taxes, your state taxing authority should have a similar option on their website.