First of all why are you trying to mark packets with IP precedence of 6? by default Cisco IP Phones mark the RTP packets with DSCP 46 (IP Precedence 5).
To aswer your question on the remarking, you can create an ACL that matches all voice VLAN RTP UDP port range and remark it with IP precedence of 5, such as:
ip access-list extended RTP
permit udp any any range 16384 32767
class-map match-any CGC-VOIP
match ip dscp ef
match access-group name RTP
policy-map VOIP
class CGC-VOIP
set ip precedence 5
priority percent 25
class CGC-VOICE-CONTROL
bandwidth percent 5
class class-default
fair-queue
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Chris