Allow 2 times schedules same day of week

Allow for 2 seperate times on the same days. We have a busy spring with our plant business. We want to be able to allow customers to choose pickups from 9-12 on Saturdays, but later in the summer we want to make less available time. we want to go to 9 to 11 on saturdays. I understand that we have to currently wait until the new period to scheudle the new time as it wont allow 2 seperate times. The issue is people order weeks or month in adavance so it would be better if we can change it now. We may have people who choose a date in July right now and they can still chose 12pm.