Can I edit my opportunity once it's published?
Yes, you can. Actually, you can edit most fields throughout the life-cycle of the opportunity.
Categories are the exception - they will be locked as soon as you add a judge to your opportunity (judges belong to categories). Please also read Editing Rounds.
Since we add you as a judge as soon as you create it, so chances are the categories will be locked as soon as you've created the call. You will need to delete all judges before you can add more categories. Categories are then permanently locked as soon as you get submissions.