I think so... haha

As long as the other SLA's are all using the same schedule then you just need to update that one schedule with the new holidays.

Always best to do this in a Dev instance first but you can add a test entry i.e. today as a holiday to ensure the SLA's don't run as a test.

But if each SLA is using a different schedule (should be rare) then yes you would need to update each schedule

Kind Regards

Ashley