according to
http://www.juniper.net/us/en/local/pdf/implementation-guides/8010073-en.pdf
you can change the dscp only in the following ways:
Remarking:
Remarking involves changing the QoS priority markings (802.1p or DSCP) for the next hop to act on.
- -Interface specific rewrite: Binding a rewrite rule to the interface.
- -Multifield remarking: Using egress firewall filters to remark specific traffic bases. This can only be applied to an L2/L3 physical or logical interface. Multifield remarking firewall filter cannot be bound to a VLAN.
Unfortunately for you ingress MF is not mentioned here
regards
alexander