You can change field labels but not field names (slugs) without losing the connection to previously entered data.
For example, if you have a text field with label “Camera Lens 1” and field name “camera_lens_1” you can change the label and the new label text will be shown in the templates where this label is used. But if you change the field name it will not update the old data in the database, so you would start with an empty list again. The old data isn’t lost, it’s still in the database, but it won’t be accessible unless you change the name back to the old one.
I hope I’ve understood your question correctly. For more specific answers you’ll have to post your field setup exactly.
