Hi Christie,
You are going to love the Candlelight Processional - it truly captures the spirit of the season like nothing else!
As the
Candlelight Dining Packages are classed as a Dining Event and require two Table-Service Dining Plan meal entitlements per person, you would not be able to simply change your existing dining reservation.
You would need to make a new dining reservation for this Dining Event via the Walt Disney World website or by calling (407) 939-3463. If there is availability, you would then need to cancel your existing dining reservation.
You would be permitted to leave the theme park after your meal and return later that evening for the Candlelight Processional if you wanted to although as you say in your question, there are certain times of year like during the holidays when the parks are extremely crowded. Whilst it is impossible to predict if a park will reach capacity on any particular day, if a park does reach capacity and is closed, no further guests will be admitted to that park.
There are four levels of closure at the Disney parks so I would recommend that you stop to ask a Cast Member before you leave Epcot if it has already or is likely to reach capacity. You will then be able to make a decision about whether to leave the park or not based on the information provided.
Wishing you a magical trip filled with memories which will last a lifetime!