Hey there @Yellow TV - great question!

Firstly - if you were to change your overall Availability Schedules right this moment it would not impact your bookings in any way, shape or form. All previously booked times/dates that are already under your “scheduled events > upcoming” tab and on your connected “add to” calendar would remain there as is - no issues at all. None of your invitees would have any idea your upcoming availability had changed and would meet with you as planned (hopefully!). 

As far as updating your availability from a specific date onward goes: yes and no.

Using “weekly hours” this is not possible, as those times apply from week to week without exception and cannot be set up twice within an event type. However, using “date specific hours” this is absolutely possible. All you have to do is use date specific hours to set up availability for the desired dates. For your use case, this could look like: 

1. Leaving your weekly hours in place for the next two weeks (if you still want to allow bookings for the next two weeks during those previously set hours)
2. Setting up date specific hours starting from April 15th through - say - May 15th
3. Come April 15th, delete your weekly hours entirely and start over with them - setting up your new availability as weekly hours (now that it is April 15th) as normal - and this will apply indefinitely or until the date range you set in place in the event type settings

Here’s a Loom I made just for you showing you how to do this and meet that goal! 

I hope this helps!