Changing an award booking

Hi FTers

I've finally saved enough miles to make some award bookings, but I'd like to check a few things.

If I make an award booking, then later see a better option (e.g. different routing or carrier), can the UA agents cancel the old booking and make the new one at the same time? I'm worried that if I need to cancel the old booking then wait for miles to be redeposited that the new, better option could disappear.

Thanks!