They don't try to amend it before the trial. They simply give the accused notification before they plea that they will be amending it up if they enter a not guilty plea. This also lets the JP know that the accused was fully informed about the possibility of an amendment.
This is from a lengthy thread earlier this year:
http://www.ontariohighwaytrafficact.com/topic6956.html "As per the Winlow decision, the prosecutor must give you the Winlow Warning (notice) BEFORE you are arraigned (i.e. enter your plea). While the prosecution can certainly notify you of their intention to 'amend up' (Winlow Warning) anytime before you plea, in practice they usually wait to see you in person---either at your resolution meeting or on the day of your trial. If they notify you on your trial date, then you are entitled to an adjournment (so as to re-consider your legal options in light of the change in circumstances). That's an entitlement set out right from the Winlow decision.
In practice, this is how it usually happens. Before you enter your plea, the prosecutor notifies you and the court that they anticipate they will be seeking an amendment to raise the rate of speed on the certificate based upon the evidence that is given. That lets the JP know that the prosecutor is planning to seek an amendment the moment the officer testifies to a higher rate of speed. The JP then asks you if you are aware of what that means and may offer you an adjournment to re-consider your position. If you turn down the offer to adjourn and say you want to proceed, then they will arraign you on the charge of speeding as it is stated on your ticket. Once the officer testifies about the higher rate of speed, the prosecutor will then move (request) to amend the certificate to the higher rate of speed.
Now, to address your other question, there is nothing that prevents you from simply entering a plea of guilty BEFORE the certificate is amended. That is, if on your trial date the officer shows up and the prosecutor gives you your Winlow warning, you can still plead guilty before they amend up. Of course, it is always wise to let the prosecutor know that you'll be pleading guilty so that they don't attempt to make the amendment BEFORE you plea and to hopefully allow you to get out of court early. Otherwise, you could be sitting in court all day while they go through their list, under the assumption that your case will be proceeding to trial.
However, keep in mind that if you ARE planning to plead guilty, then it makes more sense to simply pay the fine before you even have to enter a plea. That's because if you simply pay the fine, the amounts used are the 'set fine' rates. If however you plead guilty in court, then they must use the 'statutory rates'; which are always higher."