Hello, Nicola!
I called Disney Dining at (407) WDW-DINE just to check on this for you and was told that, just as when adding a Dining Plan to an existing reservation, you may upgrade from your Quick Service Dining Plan, but it has to be done at least 72 hours prior to your check-in.
At that time, you'll simply need to pay the difference between what you paid for your Quick Service Dining Plan and what the Dining Plan costs instead. This price may vary depending on the dates of your arrival, especially if you are traveling around a holiday, and if there are any children in your party. As an estimate, however, I looked at the current offer of a 6-night, 7-day vacation package which includes the Disney Quick-Service Dining plan, which costs $102 per person, per day. A similar package at Disney's Pop Century Resort, which includes the Plus Dining Option (rather than the Quick-Service Option) costs about $160 per adult per day.
I'd talk to a Cast Member at your earliest convenience in order to get an exact cost of your upgrade from Quick-Service Dining to the Plus Dining Option. Be sure to make your choice early enough so that you can get the upgrade.
Don't forget! Table-Service reservations can be made up to 180 days in advance of check-in, and have gotten quite popular as of late. You can make your
Table-Service reservations online or via phone, (407) WDW-DINE, even if you have not yet upgraded to the Dining Plan. We always book our dining reservations at the 180 days before check-in mark, even though I wait to add Dining until later, just to be sure that we get the reservations we need for our vacation.
Send more questions as you think of them! I'd love to help!