I have been thru this process, Its simple, you should meet all the cretira in the LC which you will be using, like Qualifications should be equal or may be more as mentioned in the LC which was approved for person X, you skill set should match with that, you current salary.
Lawyer will Apply for substitution on your behalf while filing I-140 and then I-140 will be approved in your name.
Well if there's an approved Labor Certification w/ your employer which matches yours[Job Details, Salary etc] then you can go ahead with that substitute labor to file ur I140/485..!
Lawyer might know more info on 'substitution labor' process..
People out here have got approved thru this process..!
One more question.. how exactly is the "Substitution" done at the I140 stage? Is there a special form or something that iforms INS that we are filing for a substitution. Or will INS figure out its a LC substitution on its own and then act accordingly.