How to modify the prefix length on existing dhcpv6 pool and prefix configs?

[Arutkavi]

I have a testing router Cisco model 7200/7204 model and my dhcpv6 configuration are as below.

ipv6 dhcp pool dhcpv6
 prefix-delegation pool ipv6_pool1 lifetime 7200 300
 address prefix 2001:470:19:417::/64
 dns-server 2001:4860:4860::8888
 dns-server 2001:4860:4860::8844

interface GigabitEthernet0/2.20

 ipv6 address 2001:470:19:417::1/64
 ipv6 enable
 ipv6 dhcp server dhcpv6

ipv6 local pool ipv6_pool1 2001:470:F8EE::/48 64
ipv6 local pool ipv6_ppp 2001:470:19:400::/58 64

I want to change my DHCP pool from /64 to /48. When I am trying to change the configurations it is showing error as

(config-subif)#ipv6 address 2001:470:19:417::1/48
%GigabitEthernet0/2.20: Error: 2001:470:19::/48 is overlapping with 2001:470:19:400::/64 on Virtual-Access4

Help me to configure the DHCPv6 pool and prefix configurations.

Thanks,

Arutkavi