Of course!
We received our LDN case number via email first on Friday, April 21st. My husband called to book his medical on the following Monday and got his scheduled for May 5th. When he called to book his medical, that was the earliest date they had available. He had not completed or submitted his DS-260 when he booked the medical. He has since submitted the DS-260 because you need to do so in order to schedule your appointment.
You are able to reschedule/cancel your medical up to 3 days in advance of your scheduled date, or after for a fee of 100 pounds.
After my husband submitted the DS-260, he went to the link to schedule his interview appointment. It is separate from the DS-260, so don't worry about triggering anything. There were no available appointments so he checked back every few hours. The following morning there were appointments available on several days and he scheduled his for May 23rd.
Hope this is helpful!