I think you answered your own questions so I am not exactly sure what you need.
I you want the ticket then you need to find the required points and get them into your United Account.
They seem to set a deadline of Thursday midnight CST to get the points to United.
If you decide later you want to change your flight and are more than 21 days out you should be able to do that for free.
If you want to redeposit your miles you may need to pay a fee - see this chart -
http://www.united.com/web/en-US/cont...ticketing.aspx