Well, all this "starting fresh" stuff is good for handicapping him against Gym Leaders, but come Tournament time, Ash will have learned enough of the "way we do things in the Colonies" to be able to muster his previous levels in the Tournament.
Actually, this would be the best way of showing his overall progress; that he's able to do the same with a whole new system as with his old teams and methods. i.e. he could adapt.
In fact, some say that the reason why Ash didn't progress in Hoenn was because he stuck with his Hoenn team for that tourney. Similar situation will apply here.
If he doesn't make the Quarters, the writers will have to be very convincing as to why. Conversely, him getting into the Finals would be seen as "too easy" this time around.
Of course, since most Leagues are single-elim tournies, a mis-matched seeding may be a valid justification. Heck, Ash might be due for that sort of Diabolus ex Machina. Just don't overuse it. Some guy getting a "lucky shot" is also realistic, but less plausible. Then again, there's the Diabolus of Pikachu getting an injury.
The sum of it is that often times the "best" person doesn't win long drawn-out single elims, just the person with the best "draw". (Can I get an amen from veterans of PS2's Challenge Cup?) Heck, Ash, and even Ritchie, might have had a "soft draw" in Kanto, whereas Gary might have just hit a wall in Round 4. Also, not everyone plays 100% 100% of the time.
Just don't give us another guy who uses Legendaries! By Arceus, you make it so that Legendaries are revered as gods, then you give us a trainer that has at LEAST two!