Another pair of shoes ???

My English teacher used this term to express this meaning: It's another matter.

Ex: This survey's objectives are very good. But, in term of how to conduct the survey, it is another pair of shoes.

Is it acceptable? If you were my teacher, what expression would you use ?

Thanks in advance.

PLease help me, teachers !!!

It is a different kettle of fish. ;-)

That's what I want !!! Thank you very much !!!:-)