Hi - just to reiterate and clarify what others have said -
1) Job duties and responsibilities are absolutely REQUIRED in the letter of employment. Without that, CIC will have no way to evaluate your work experience, and your application will be refused. CIC will look at the job duties in the Letter of employment and determine whether you have experience in the specified NOC.
2) I frankly can't tell from your explanation which NOC you believe is the correct one. But note that NOC 8253 is a SUPERVISORY position. If you don't supervise other workers, then it is the wrong NOC. It may not be that simple in real life, but that is how CIC will view it. Similarly, 8252 is a managerial position. If you do perform supervisory or managerial duties, then you must make sure that your new letter of employment indicates that.
3) You would not be the first person to have variances in the NOC code assigned by your employer, on the LMO, and on the work permit. It's an inexact process, to be sure. It becomes a problem if the work permit is issued for a higher skilled position than you actually do - Oddly, 8252 and 8253 are both NOC level B (managerial positions are usually NOC level O, so this is strange), so the difference between the two isn't going to be an issue. (It might be an issue if CIC decides that you were really NOC 8431, or Skill level C).
Bottom line - when you submitted your application, you did not follow CIC's instructions, because your letter of employment did not specify your duties and responsibilities. You are worrying about the "wrong thing" in this case - yes, getting the NOC correct is very important, but without job duties and responsibilities in your letter, you essentially have no NOC.
If you have submitted all your required documents and on the surface appear to meet the CEC program requirements, your application will be put into processing - you will receive your "AOR" (acknowledgement of receipt) from CIC with your file number. After you get your file number, you can submit additional documents (such as a supplemental Letter of Employment that includes job duties and responsibilties). But until you receive your file number, CIC's central intake in Sydney has no way to match your new letter with the rest of your file. So wait until you get your AOR to submit a supplemental letter (it's not really an "update" since you can't add new experience to your application).