well, in a CIP country with a proof house its not possible. a CIP conform 7x57 chamber reamed out with a 7x64 CIP conform reamer will give a step and the proof house will find it.
there is a special cartridge, the 7x67SR Luyven http://old.municion.org/7mm/7x67Luyven.htm

to do such a change only this cartridge is recommanded here