Makes a lot of sense! Only one station needs rebuilding, which is Pearson. Unions UP terminal is already going to be abandoned, and the raised portion can just be removed from the other 3 stations as easily as they were installed.
Transitioning to low floor also allows UP to stop at additional stations once it’s electrified, without any additional infrastructure.
King-Liberty, St. Clair-Old Weston, and 1 additional station along the corridor are all potential stopping locations for UP post-electrification.
As said above the UP Express is becoming more or fully integrated with GO, which includes potentially interlining it with a service in the east, this can only be done with low floor trains.