From what I've heard through the grape vine, Great Gulf has put these towers on the back burner for the time being. Right now they are expecting to launch sales in 2020, construction in 2021. So it may be a while yet before we hear any news.
Essentially the previous official plan amendment required heritage preservation and placed a maximum density and height limit on the project. An OPA means they are either trying to drop heritage elements, or increasing fsi/height. I suspect it's the latter.
If it was a use change it would just be a rezoning.
Can’t see the city giving them any more density after the fight they had years ago which clipped them down to existing approved height. Maybe they would rather do three smaller towers to mitigate risk.