I understand that you have upgraded your Pega Customer Service application from 7.22 to 7.4.
I am assuming that you mean the below expand/collapse button that you see next Add Task.
This UI design has been changed in Pega CS 7.31 itself (similar to CS 7.4). The general reason for such re-designs could be for improved user experience (UX). Please let me know if this answers your query.