Yes you can upgrade - we do that from the regular to deluxe. You would just pay the cost difference per person per day.
The Standard Dining Plan is $55.59 per adult and $17.16 per kid most of the year and $56.94 per adult and $18.16 during peak season.
The Quick Service Dining Plan is $37.58 per adult and $14.32 per kid
So, during peak season it would be $19.36 more per adult per day
I took this information from here
************************************dining-plan-2013-disney-world-costs/
Hope this helps!
Julie