Well......yes and no.
Since well before Metrolinx was ever a gleam in any politician's eye, GO has been quite allergic to changing train sizes. They would only do it if they absolutely needed to - and under no circumstances would they do so just to satisfy a one- or two-day need.
(I will add the caveat that they did for a while with the bike trains to Niagara Falls, but even then they would do some funny tricks like putting together a 6-car sub-set of equipment that they would then remove from a train and leave aside for the entirety of the work week. But towards the end of pre-COVID they were going way from this model too, and had just set aside full 10- and 12-car trainsets of equipment in the yard for the week.)
So if a 10-car train is to be used on a weekend, it will have to be from a set that is 10-cars during the week. And that might mean bring trains in from Georgetown, instead of from Willowbrook.
Dan