Yes if you are only upgrading one way then you only use half of the mileage amount.
The only time we have ever had any problems getting companion seats - which are classed the same as upgrades, is when we wanted them for the October half term.
Basically, VA are not releasing hardly any mileage seats during school holiday periods. You will normally find that there are seats available 2 weeks before and 2 weeks after a holiday period.
When I booked the seats for this December, because the children are being taken out of school, I was able to get 3 companion seats and 1 mileage upgrade.
I booked these as soon as the return date became available.
You can check to see what seats are available by logging into your account on the VA website and then putting in your dates. If you want a mileage upgrade or companion seat you need to select "use miles" and it will show you what is available. It will show you all the dates that are free. You can also see how much miles plus money fares will be and if there are any available.
The reason Terry is having problems with his seats is that there aren't many UC seats on the Orlando flight, so they tend to hold on to these as long as possible with the hope that people will pay for them. When it looks like they are going to be empty then they release them to club members for upgrades/companions.
Hope this helps