We had CoS put on our link between two endpoints on the PER. Why would the need to schedule a brief outage to have that done? They said the link would need to come down in order to apply this. What is it they do on their end exactly?
it is difficult to say I suppose that shutting down the link is part of their procedure
on PE side they just need to apply a service-policy inbound to accept your marking and to drop and/or marking down non conforming traffic.
On cisco equipment this doesn't need to tear down the link, and likely it is not needed on other vendors' equipment
For this reason my guess is that it is a part of a procedure they follow not a real technical need.
Or they prefer to do even a small change under the umbrella of a declared maintanance window to be on the safe side.
Hope to help