Upgrading MYW Park Hopper Tickets

[klh999]

Ok..

So we are doing the MYW plus free dining. Half of our party is only staying for four days and the other half for six days. Plus I have tickets each, that have remaining days on them.

Can I book a MYW with just a park hopper for one day and then upgrade when I get there for certain guests and not others. Can I even upgrade once I get there? If I upgrade how do they figure the difference??

Thanks Kristy