You can call in and cancel the scheduled payments:

To revoke or cancel any EFW payment, contact the U.S. Treasury Financial Agent, toll-free, at 1-888-353-4537. Cancellation requests must be received no later than 8:00 p.m., Eastern Standard Time, at least two business days prior to the scheduled payment date.

Then use DirectPay to make higher ones...they only let you schedule 30 days in advance through DirectPay though, so its kind of a hassle.