Hi and welcome to the site.
You are correct, to upgrade for PE to UC you will need to buy an S or W PE fare and then need G UC reward fares to be available, which you can find out by calling VS or by doing a dummy miles booking on the VS website.
Now comes my negative head. To be brutally honest, trying to get G UC reward seats to Orlando at any time of year isn't the easiest of tasks, yet alone during the more popular periods. While your dates are outside the main Easter holidays, April is generally a very popular time for travel to MCO and I really wouldn't get your hopes pinned on there being availability.
As it seems you are fixed on your dates (being flexible really could help) I would suggest you have a look at flying in to Miami and driving/internal flight to MCO or maybe somewhere like JFK and then an internal to MCO, these flights have more UC seats on them so availability is usually greater for reward seats (MCO flights only have max 14 seats and some don't have any UC at all).
I hope you get lucky and my negative post is all incorrect, but as a regular traveller to MCO, throughout various times of the year, I think it was best to offer an honest view rather than you having your hopes built up only not to be fulfilled.
Thanks,
Neil