My apologies if this is a FAQ somewhere.

I'm looking into booking a United flight with Krisflyer miles. I am finding some flights around the dates I need to travel but not the exact dates I wanted. Can I book the flight first through the Singapore website and then change flights with United directly to the dates I want after that fact? Or am I not allowed to change the flights in any way?

Also, does the "Economy" class for United map to United's basic economy or regular economy? TIA.