What would have happened if a trainer stayed with a certain fighter?

[J_Undisputed]

Depends on the Fighter. Some fighters just respond to certain trainers better. There are the types of trainers that push a fight to be the best because they know what you are capable of and won't stop till the fighter realizes it...no matter how much they whine and complain.

Theres the other trainers that don't want to take the fighter out of their comfort zone, because they feel its creates mental lapses and cracks in their confidence which may give them some desired results but created problems elsewhere.

It all depends on the type of dedication the fighter will give to his trainer and the focus on his instructions. Tysons trained with Roach for a fighter, Tommy Brooks, Tried out with Mcgirt... and others who are good trainers...but never did what he was supposed to. Some fighters have an ego and think because they sign the checks, they are the boss. The trainer is the boss, and sometimes a fighter needs to get clipped in order to see that and start following instructions. Other times, they'll never learn no matter what and will always look to blame their trainer (de la hoya). They funny thing is when a fighter gets a mentality that because they are taking the punches, they have more at stake... and fail to realize that when a person trains you for hours at a time, watches tapes, makes adjustments, checks up on you to make sure you're head is in the right place and pretty much acts like a mentor.... They have a lot at stake too; maybe even more... they preparing sometimes long after the fighters left the gym... gone to bed...

-De la Hoya fought very much the same way, no matter who the trainer flavor of the month was.
-Manny Steward has Vlad on auto pilot. He already comes in better shape than most of his opponents care to be in... and none of them put any work in to slipping the jab or putting combinations together. Manny could sit in the corner and and read a magazine and go jab ... jab.. jab.. thats right.. come over with the right...left hook...good.. hands high... Step.. circle... jab jab... and it will pretty much work for anyone Vlad fights. he'll tweak a little for anything particular habits of an opponent..
- Make or break instances... Judah and Tua leaving Ronnie Shields and Lou Duva.
Corrales getting with Goosen
Tyson leaving Rooney (of course)
Pac Training with Roach (of course)
Theres probably a billion other instances..

[Nameless]

I don't know in term of performance but Tyson would have been quieter for a longer period of time, possibly staying out of the big troubles for an extended lapse of time.
I think Wlad could have been more or less as good with Roach as Freddie doesn't train all his fighters the same way, he would have spotted Wlad's weaknesses and strengths and adjust in consequence. I can't say he would have done a job as good as Manny but it would have been, in my opinion, a very great job indeed, if the chemistry would have been ok between the two, of course.