Great question. I only have formal rules for draft tournaments,so this rule is not currently clarified anywhere else. Yes, in constructed tournaments (As you described) you will be able to modify your deck after seeing your opponent's line up of gruffs/shepherd.
You could also use the locked-in decklist + sideboard and do the best of three mode.
You'd come to the tournament with a full constructed deck, and a list of, say, 6-10 sideboard cards that you could change between two rounds to adapt to your opponent's strategy.
Ideas here. I haven't played enough to know if it's relevant.