Normally what happens with such procedures is that partial shutdowns are done at the intersection, but the radical method of replacing the tracks all in one pre-fab piece demands that the whole x/y traffic be stopped. The section is pre-welded and dropped whole into place which makes it an easier job than recreating the assembly piece-by-piece on location.
This is how the streetcar tracks were all changed-over at the intersection of King and Church, back in the summer of 2004. I happened to be walking by/through when the big crane was dropping the track section into place. I wished I had my camera.
Since these days I'm down at that intersection a few times a week I will make sure that I don't miss this one.
"King-Spadina intersection to close for two weeks"