Unless the twists the judges throw in when the rounds are announced say otherwise, then yes, your offerings for the subsequent rounds can (and probably should) be different.
Of course, the judges might throw a curve in and say that in the encounter round you need to include one of the finalist's monsters from the prior round.
Contestants have been criticized for not breaking out in the past. I also, think Neil has answered this by saying why would you want your potential employer to think you are a only good at one thing?
The rules dinnae* say one way or the other, but the twist usually makes it difficult. The encounter round for example, will often require a monster from the monster round that isn't your own.
*Round rules not yet being posted of course.
EDIT: ninja'd
If you look at prior years, one of the contestants got pretty far with all sea themed items, their encounter was on an island that was re-used in their module pitch.
You can see from around the third round, even the general public were worrying it was a one trick pony.
You can also see in past competitions, that occasionally the judges place a twist on a round (e.g. in this encounter round, pick one of the monsters from the prior monster round and use it as the central piece of your encounter.) So linking of rounds can also happen in the competition rules.
So as Curaigh said, you should look to exhibit a wide range of skills and areas in the art of adventure writing and designing should you progress from round to round.
To summarize the post works Xmas party waffle...
Each round, be original, make it cool = get my vote.