Hi @peterf
When you save / load a value, ACF will use the field name to find the value in the wp_postmeta table. This is pretty basic WP metadata stuff.
If you change the name, ACF will now be looking in the wrong place for the value.
You can always edit your database and change all the meta_key references from {old_name} to {new_name}.
Good luck
Cheers
E
