You don't necessarily need to wait for the CMTS side of your modem to use IPv6. My SB6121 is still using IPv4 on the cable side, but I nonetheless have full native IPv4/IPv6 connectivity.
C:\>ipconfig
Windows IP Configuration
Ethernet adapter Local Area Connection 2:
Connection-specific DNS Suffix . : dcs-net
IP Address. . . . . . . . . . . . : 192.168.9.100
Subnet Mask . . . . . . . . . . . : 255.255.255.0
IP Address. . . . . . . . . . . . : 2601:5:c80:91:493d:2c4:6e8d:1572
IP Address. . . . . . . . . . . . : 2601:5:c80:91:e291:f5ff:fe95:b69d
IP Address. . . . . . . . . . . . : fe80::e291:f5ff:fe95:b69d%26
Default Gateway . . . . . . . . . : 192.168.9.254
fe80::1e7e:e5ff:fe4c:e6ff%26
Tunnel adapter Teredo Tunneling Pseudo-Interface:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : fe80::ffff:ffff:fffd%5
Default Gateway . . . . . . . . . :
Tunnel adapter Automatic Tunneling Pseudo-Interface:
Connection-specific DNS Suffix . : dcs-net
IP Address. . . . . . . . . . . . : fe80::5efe:192.168.9.100%2
Default Gateway . . . . . . . . . :
C:\>netsh int ipv6 show addr
Querying active state...
Interface 26: Local Area Connection 2
Addr Type DAD State Valid Life Pref. Life Address
--------- ---------- ------------ ------------ -----------------------------
Temporary Preferred 3d9h28m11s 8h12m8s 2601:5:c80:91:493d:2c4:6e8d:1572
Public Preferred 3d9h28m11s 3d9h28m11s 2601:5:c80:91:e291:f5ff:fe95:b69d
Link Preferred infinite infinite fe80::e291:f5ff:fe95:b69d
Interface 5: Teredo Tunneling Pseudo-Interface
Addr Type DAD State Valid Life Pref. Life Address
--------- ---------- ------------ ------------ -----------------------------
Link Preferred infinite infinite fe80::ffff:ffff:fffd
Interface 2: Automatic Tunneling Pseudo-Interface
Addr Type DAD State Valid Life Pref. Life Address
--------- ---------- ------------ ------------ -----------------------------
Link Preferred infinite infinite fe80::5efe:192.168.9.100
Interface 1: Loopback Pseudo-Interface
Addr Type DAD State Valid Life Pref. Life Address
--------- ---------- ------------ ------------ -----------------------------
Loopback Preferred infinite infinite ::1
Link Preferred infinite infinite fe80::1
C:\>ping www.comcast.net
Pinging a1526.dscg.akamai.net [2001:559:0:5d::1743:3d3b] with 32 bytes of data:
Reply from 2001:559:0:5d::1743:3d3b: time=40ms
Reply from 2001:559:0:5d::1743:3d3b: time=31ms
Reply from 2001:559:0:5d::1743:3d3b: time=32ms
Reply from 2001:559:0:5d::1743:3d3b: time=30ms
Ping statistics for 2001:559:0:5d::1743:3d3b:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 30ms, Maximum = 40ms, Average = 33ms
C:\>ping -4 www.comcast.net
Pinging a1526.dscg.akamai.net [184.84.220.27] with 32 bytes of data:
Reply from 184.84.220.27: bytes=32 time=19ms TTL=58
Reply from 184.84.220.27: bytes=32 time=19ms TTL=58
Reply from 184.84.220.27: bytes=32 time=19ms TTL=58
Reply from 184.84.220.27: bytes=32 time=19ms TTL=58
Ping statistics for 184.84.220.27:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 19ms, Maximum = 19ms, Average = 19ms