I already have 3 x UC seats on this route for Christmas booked with miles and am thinking of the same route for April. I haven't enough miles for three seats however and was thinking of booking 3 x PE with a view to upgrading with miles to UC. There does seem to be so much more availability on this route than to MCO. Do you think this would be a good way of securing UC seats?
As long as you are prepared to fly PE should no UC seats become available then yes, it's a fine way of doing it.
Bear in mind it's Easter in April next year and VS will be very aware that many people will look to use ATL as a way to get to MCO over the very busy period.