According to a report on Thursday on German news site, Motorsport-Total, di Grassi is thought to be planning to race with the Audi Sport ABT Formula E team, because of his existing connections with Audi through Le Mans racing.
This leaves an opening at Formula E for a test driver. According to CurrentE, the Formula E office didn't confirm di Grassi's departure, but did confirm the rules quandary that would force di Grassi to leave.
Apparently, "The Stig" (Ben Collins) has said he's going to join Formula E racing - as a development driver
Delighted to sign with @FIAformulaE development program & ready to unleash high voltage! @autosportnews @vwmotorsport @ST_Driving #electric
— Ben Collins / Stig (@BenCollinsStig) December 12, 2013
@Current_E @ofux1 @FIAformulaE no premature annunciation - but more official info to follow!
— Ben Collins / Stig (@BenCollinsStig) December 13, 2013