Hello Rod
It is always possible for an administrator to change the DSCP values of traffic that goes over their own network. It is only natural that you have power over such parameters when it comes to traffic traversing your networks. There are many reasons why an ISP would do this.
For example, the SLA of the ISP may state simply that they will route all customer traffic on a first come first serve best effort basis. This means that all traffic will be treated equally. Internally, the ISP may have configured QoS mechanisms for their own traffic (control traffic or otherwise) so they want to make sure that those mechanisms don’t act upon customer traffic, thus possibly causing disruption to their own prioritized traffic.
Now if you choose to, you can request some level of QoS on the ISP’s network for your traffic, but this may cost more, and it depends upon the policies of each ISP. But remember, once traffic leaves their network and enters the Internet at large, even if packets have DSCP values marked, QoS is virtually non-existent there, and everything is routed as best effort.
Even if your ISP keeps the DSCP values, once they route the traffic to a Tier-2 or a Tier-1 ISP, the DSCP values are typically reset, or simply ignored. For more info on the Tier structure of the Internet, take a look at this post:
I hope this has been helpful!
Laz