If it’s feasible for you, you can just shutdown all processes at once (or close to it). There shouldn’t be any significant harm in it being more staggered, but that will result in the movement you are trying to avoid.
Another option to avoid movement if you are working on one fault domain at a time is to use the maintenance command in fdbcli (available since 6.1). This will mark a fault domain as in maintenance and won’t replicate data from it if it goes down. I don’t see any documentation on our website for it, but you can run help maintenance in fdbcli.