It is not possible to remove or change the Network Address Translation (NAT) configuration if there is an active NAT in the NAT table.
Clear the NAT table prior to making any changes to the NAT configuration by choosing any one of these options:
Issue the clear ip nat translation command and then replacing the NAT configuration quickly, before any new NAT entries are populated into the translation table due to active NAT traffic. To do this, create a script with the configuration commands written in a text format. For example:
clear ip nat translation * config terminal no ip nat pool old pool name
ip nat pool new pool
Disable the NAT on the router by removing the ip nat inside or ip nat outside commands on the interfaces. Then clear the translation table and change the configuration.