@Niter thread got me thinking. Was going to MK on Oct 3. so made FPPs. First one is for 3:55-4:55. Changed our minds and are now going to Mickey's Halloween Party only. Will get there at 4. Do I need to change my FPP so not to be charged for full day?
No, you should not be charged for the day. Just make sure to use your party tickets to enter the park after 4:00.