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.