Replaces the message content of an existing broadcast. References the sending domain and footer by UUID. Either html_content or raw_html_content must be supplied. If from_email or reply_to_email are supplied, their domain must match the sending domain of the referenced EDS. The body carries an offer link as the offer link variable of an offer linked to this broadcast, pasted exactly as returned with its braces -- {{offer_link_4821}} -- which resolves per recipient at send time into the tracked link, carrying the attribution token whenever the offer's tracking URL leaves a spare attribution parameter. A tracked URL pasted into the body instead never carries an attribution token, so conversions cannot be tied back to the contact. The variable comes from a create_broadcast, set_broadcast_offers, or get_broadcast response; when it is not already known, link the offers first and place the variable in a follow-up call to this endpoint.