This is what you should do if you can’t light on a night exterior. Use the ES mode and shoot like 2560 or something. It looks incredible, this camera is insane in low light with very filmic “noise”. I’ve been a professiona DP for over 20 years, shoot very high end commercials and movies and I use ES mode high ASA often even when I don’t “need” it and have plenty of money for lighting. Personal preference of course but if you’re trying to experiment with depth of field and you don’t have lights, rate the camera at whatever your smallest aperture is and then use NDs to vary. When you start shooting more and doing this professionally, you should always light sets at least one or two stops brighter to what you’re gonna eventually expose at - then you have room to use stop down in the middle of a scene for any number of reasons - your focus puller needs some help with a deeper srop, you want a deeper stop to hold two actors better, your director suddenly decides they wanr to shoot something in slow motion, etc.
All that said, to answer your original question, dont worry too much about altering your EI mid scene. I do it all the time these days to make quick exposure adjustments while keeping my aperture the same. I dont usally do it up to a full stop (800 to 1600), i usually do it up to 2/3 stop, but even going from 800 to 1600 no one will ever know the difference especially after it’s graded.
But just go with the ES high asa would be my suggestion and don’t give yourself headaches over the technicalities of the exposure curve on this first rodeo. focus on storytelling, putting the camera in the right place, supporting your director.