Inspired by this answer and it's comments.
Early on, it was standard to refer to great Rabbis who had passed away with the appellation "Zal" - "zichrono/ah livracha "May his/her/their memory be a blessing" (see here). Over time, people started using the appellation "Zatzal" - zecher tzadik livracha "May the memory of this tzaddik be a blessing" - when talking about great rabbis who have passed away.
When did this trend start? When was the shift made from "Zal" to "Zatzal"?