Importing, or updating a field group or changing a field will have no effect on any data that is saved to those fields on posts, or wherever they are saved. That data will still be there. If you have changed a fields properties then the values in those fields may or may not continue to work depending on the changes that were made.
It’s hard to say what will continue to work and what won’t, it really depends on what the field is, how the value is stored and what is done with the value for display.
