I think you need to look at the script and locations and work out if there are locations and situations that look different anyway that could be shot digitally.
For instance in might be possible to shoot all the night scenes on digital or alternatively to shoot scenes in some other context digitally.
You want to make sure that you don't end up directly intercutting the film and digital footage, so make sure that the digital footage is in new scenes. Even better if you can cut to black inbetween.
If you are lucky you might even be able to make the changes in look motivated in some way, for instance if there are flashbacks in the movie then the flashbacks could be shot on digital or visa versa.
If there are scenes that require more footage and they are already shot then it would be best if those scenes continued to be shot on film so that it is consistent and you aren't trying to match film and digital shots in the same scene which makes things even harder.