The programming has to include the accounting, (charges to the Feds, new charges to the states), accounting for future overpayments, date limitations, etc. Not an overnight change.
If states weren’t pathetic incompetencies, that wouldn’t be the case and yes, it would be an overnight change.
Not that difficult - my wife works for local govt, and has to submit FEMA invoices all the time.
Basically, states request the assistance and when authorized begin making $400 payments to eligible state residents. Each payment the state makes, has a date - then bill FEMA for 75% ($300) for all payments between Aug 1 - Dec 6, 2020