Who Goes First

At Theros Game Day we had an issue regarding who plays first.

The players shuffled & cut and agreed to high roll. Evan, won the high roll decided to go first, then looked at his hand and mulliganed. Andrew decided to keep.

Evan reshuffled, cut, and drew six cards. 

When Evan looked at his hand the second time he wanted to go second. 

Andrew did not think that this was fair--Evan had already decided to go first.

My understanding is that you have to choose who is going first before you draw your cards, is that true?

Once that decision is made, can it be changed?

I ruled that Evan could go second, but mostly because I did not really understand why it mattered & wanted to keep things moving--I do not think that this would have been the Official Tournament ruling.