Nothing has changed; your reservation has to be ticketed before you can buy additional RDM and PQM. Because of the way UA's IT works these days, ticketing doesn't occur at time of purchase (though it's usually within 10 minutes or so afterwards).

Once the reservation has been ticketed, you will have the "buy extra miles" link on the reservation, and can examine your RDM and PQM offers. Be aware that right now they are very pricey since we're approaching the end of the year (any offers you buy, even for travel booked for next year, will count for this year, BTW).