There is no automated way to move the data from one to the other.
Data is stored in the _termmeta table by the term_id. When you create a new “category” this is a new term with a new term_id even if the name/slug is the same as the original “tag”.
Moving the data would require doing an DB search for all the _termmeta connected to the old term_id and replacing it with the new term_id.
You could, potentially, use the WP create_term hook. In the hook you could search the tags for a matching slug and then copy all of the ACF fields using get_field() and update_field(). Note that you will need to use the field keys to do the update process and NOT the field names.
