Does anyone have that IRS number I can call to change the e-payment amount due on May 17? Per instructions ending with a ?

You call the IRS e-file Payment Services automated line at 888-353-4537.  You should call no later than 11:59 p.m. Eastern time two business days prior to the date your payment is scheduled. 

Just so everyone is aware, I recently found that the IRS has no idea how to modify a future-dated payment in their Direct Payment system. I've reached out to a supervisor there to escalate the matter. Someone in that organization must know how to cancel a future-dated payment. It is THEIR system. If they cannot help me, I have to fill out an ACH Stop Payment with my credit union and pray it works. If it doesn't, and the IRS takes both posted income tax payment amounts, I'm afraid I'll never see it again. 

You can not modify the date.  You have to cancel the payment and then schedule a new payment.