If you change the % complete type then you will have to enter the % manually and also the Remaining duration for the activity.
For duration %complete, when you enter the % the remaining is calculated based on that %. Remaining duration determines the Finish date of the activity and the remaining hours for each resource.
I have not changed duration types on activities in progress, that also have resource assignments in progress. I am not 100% sure what the impact would be on their remaining hours.
Fixed duration and units/time will help you maintain the durations constant when adding or removing resources, and allow you better control of your schedule. I suggest to use the Fixed duration and units per time or fixed duration and units rather than fixed units or fixed units/time whenever you create resource loaded schedules, but I cannot really tell what the impact would be when changing existing ones.