It is possible to set a custom field group key for any field group. In order to change the key, you’ll have to display the “Slug” metabox. The field group key must begin with group_. Note that PHP & Json export files will get updated accordingly.
Group key is saved in the Field Group array, under key key. It can be retrieved using acf_get_field_group(). Usage example:
$field_group = acf_get_field_group('group_5f20935b9a777');
/*
* [instruction_placement] => label
* [hide_on_screen] =>
* ...
* [key] => group_my_field_group
* ...
*/