I played a lot of melee to practise before i got brawl, and i must say depending on the character it could be similar or completely different. I planned on maining peach but she is so radically different in brawl (for the worse imho but thats not the point of this thread
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Since most liekly everyone at the tournament will be in the same position as you (not having played the game) then you wont be at a disadvantage.
Best bet is to choose a char that hasn't changed too much between melee and brawl.
Mario or pikachu or marth is probably best to practise with in melee, those two have probably has changed the least in my opinion, and are the most likely to support your melee skills.
I would say go with pikachu, with his decent recovery you might be able to rescue some potential suicide deaths due to not being used to the new physics and handling...
When i first started brawl i could barely do anything at all without suiciding against lvl 3 comps >_>
several hours later lvl 9 comps are a breeze. The controls definatlely do take getting used to, althought thats because i use the wiimote and nunchuck (work brilliantly once you get used to them btw)