"Unable to change flowmask" error message on Cisco 6500

You will not be able to change the flowmask after you configure NAT. You might be hitting the bug CSCsb41562 (fixed on 12.2(18)SXF)

Actually it's not a bug I think it's a feature, and I'm not sure that it will be fixed. If you use the NAT you can't use full-flow for Netflow, because of some internal restrictions: NAT and Netflow uses different flow masks and they are not compatible.