"It is not clear how Scott will produce the Spacey-Plummer swap, but bringing back the actors for filming and finishing the film on this razor tight schedule is an enormous undertaking. Another, also monumental, option may be delicate visual effects work. A key challenge will the visual effects, since many of Spaceys scenes were shot in different locations.
VFX house MPC which was the lead VFX house on last years visual effects Oscar winner The Jungle Book is working on All the Money in the World (though there might be additional VFX houses involved). Depending on the complexity and number of shots in which the character appears, it is possible that digital compositing could place Plummer into some shots.
Blade Runner 2049. Techniques such as this have been used in unique situations to finish a film, though its difficult to think of another predicament such as this. "