You can account for seasonal changes in service times or ministry requirements that take place *in the middle of a schedule* by first creating a schedule skeleton with the time and / or ministry settings that correspond to the first part of the schedule. You can then edit each service that has a different time or ministry requirements by selecting its day from the calendar, clicking on the service, and then clicking on the Edit Service button. The adjust the time and / or ministry requirements as appropriate. (You can delete unneeded services completely by selecting them and then clicking the "Delete Service" button.) When you have finished making adjustments to each of the necessary services, you can then choose "Save Skeleton" from the File menu to save the changes you've made. Then click the Commence Scheduling button and the services you modified will be scheduled according to their new settings.
If during the next schedule you run, all the services will have the new settings (as opposed to just the second part of the schedule), you should make the appropriate adjustments in the Service Times heading on the left hand side of MSP's main screen. That way you will not need to manually edit each instance of the services in the schedule skeleton window so that they are scheduled with the new settings.