JunkieXL is going to do his own style like he always has. Studios rarely interfere with film scores. That's left to the composer with the input of the director. Why would Snyder or Junkie alter the themes and tone and style? If you're hoping for John Williams or Dany Elfman type music you're not going to get it from Junkie.
Umm.... it's more often than just 'rare.'
- Marvel Studios scrapped half of Brian Tyler's music for
Avengers 2 and said sayonara to his Marvel fanfare when he opted to finish
Furious 7 in lieu of
Ultron. They brought in Elfman to finish things up.
- Universal took out Elfman's score for
The Wolfman, then changed their minds and put it back in. Three composers wrote additional score in Elfman's style to accommodate the recut film.
- Danny Elfman went through hell when Sam Raimi fell in love with the temp track for
Spider-Man 2 and ended up leaving during final dubbing. John Debney and Christopher Young provided additional score for the finished film.
- Christopher Young, who replaced Elfman on
Spider-Man 3, had portions of his score dropped in favor of re-recordings of Elfman's themes. (Much of his stuff were themes for Aunt May, Mary Jane, and Gwen Stacy.)
- Jerry Bruckheimer rejected Alan Silvestri's demo themes for the first
Pirates of the Caribbean and brought in Hans Zimmer instead.
- Jon Favreau was forced by Marvel to use Ramin Djawadi for
Iron Man, in lieu of his previous collaborator John Debney (who did IM2).
- Warner Brothers rejected the original scores for
Troy and
Pan by Gabriel Yared and Dario Marianelli and enlisted James Horner and John Powell for last minute replacement scores.
There is precedent for a similar situation to pop up in JL. Or it could be smooth sailing, music-wise.