Although no replacement has been hired as of yet for 'Bond 25', news is beginning to leak out as to why it was that Danny Boyle walked away from the movie.
A source speaking to the Telegraph is claiming that Boyle's exit was precipitated by a clash with Daniel Craig over a casting decision involving Polish actor Tomasz Kot. Apparently, the script for the as-yet untitled movie involves a Russian villain and updating Cold War elements in light of the current tensions between Russia and the West.
While rumours initially suggested that this was the source of contention, the Telegraph claims that Boyle backed out as Daniel Craig had final say over casting and that Tomasz Kot was the crux of the issue. It's not known whether Craig was for or against Kot, but given how the actor has yet to be announced formally by the producers, it seems likely that the decision has yet to be made.
The Telegraph's report also claims that Boyle was clashing with longtime producer Barbara Broccoli, whom Boyle had apparently vexed by bringing in his own team including longtime screenwriting partner John Hodge. The script written by Neal Purvis and Robert Wade, who have had a hand in every Bond movie since 1999's 'The World Is Not Enough', was apparently put on the backburner to accommodate Boyle, but now that he's out of the picture, their script may end up being the one used.
As for a replacement, Yann Demange - who directed the Northern Ireland-set '71' - is understood to be the frontrunner to replace Boyle.
'Bond 25' is set for an Irish release on October 25th, 2019.