Hiya, Nicole!
I am such a Type A personality, that I plan which park we should visit each day pretty much as soon as we book our trip.
You will find plenty of third-party input if you search the web for a Crowd Calender, but these sources tend to vary a lot, so I would like to let you in on my personal strategy as well.
For my family, our first day is almost always Magic Kingdom. Especially if you are a first time or infrequent visitor, walking through the gates to see Cinderella Castle is the most magical experience, and really makes you feel like you have arrived!
Other than that, I try to plan my park days so that Hollywood Studios and Magic Kingdom are book-ending our trip. I find that our busiest days are spent at Magic Kingdom and Disney's Hollywood Studios so I like to put Animal Kingdom and/or EPCOT in between those days, to help make sure that we do not get too worn out.
Another thing that I look at is
theme park hours. Like many people, I want to get the most bang for our buck, so if there is one day that Disney's Animal Kingdom closes at 7:00 pm, instead of 5:00 pm, I am going to try to make that our day on the safari.
I also like to book our trips so that the majority of the theme park days fall on weekdays instead of a Saturday or Sunday. Many local residents are Annual Passholders, so the parks tend to be a little more crowded on the weekend, especially EPCOT and Disney's Hollywood Studios.
To answer your second question, you are able to upgrade your
ticket to the Park Hopper option after you arrive, however you must upgrade your entire ticket and not just a single day.
I hope that this gave you a bit of guidance, Nicole. Please come back here to planDisney if we can help make your vacation planning experience a little more magical!
Sarah B.