I suspected the FISA abuse investigation has been underway for some time now. The announcement is probably more of an indication that the first OIG report will not cover FISA abuses. But this blog discusses all the facts suggesting the investigation into FISA abuses has been underway since July of 2017.
https://www.themarketswork.com/2018/03/29/understanding-the-inspector-generals-announcement/
I suspect that the late March 2018 announcement is exactly what it states, an initiation of a FISA abuse review.