Hunter, who originally joined the show in July last year understudying the role of Guy, and Winternitz will have their first performance together on Monday (12 May 2014).
Hunter (Guy) was a semi-finalist on Andrew Lloyd Webber's ITV casting show Superstar on ITV. His theatre credits include One Man Two Guvnors, Seussical and The Hired Man.
Winternitz (Girl) made her West End debut starring as Baby in Dirty Dancing at the Piccadilly Theatre and then on tour around the UK.
They'll be joined in the cast by: Fiona Bruce (Baruska), Jamie Cameron (Emcee) Mark Carlisle (Da), Matthew Ganley (Svec), Mathew Hamper (Andre), Daniel Healy (Eamon), Loren O'Dair (Reza), Miria Parvin (Ex-girlfriend), Tim Prottey-Jones (Billy) and Jez Unwin (Bank Manager).
The show's current leads, Olivier Award-winner Zrinka Cvitešić and Arthur Darvill, have their last performance on Saturday (10 May).
Once won Best Original Music at the 2014 WhatsOnStage Awards, and last month won two Olivier awards - including Outstanding Achievement in Music. The show has also won eight Tony awards including Best Musical and the Grammy for Best Musical Theater Album.
In addition Glen Hansard and Markéta Irglová's "Falling Slowly" won the Oscar for Best Original Song.
Directed by John Tiffany, the West End production of Once opened in April 2013 and is currently booking at the Phoenix Theatre to 4 July 2015.
Based on the 2006 film, Once is a celebration of love, friendship and music. When an Irish busker and a young Czech mother meet through a shared love of music, their songwriting sparks a deep connection and a tender, longing romance that neither of them could have expected.