The forms that require only numbers instead of letters usually do so because the information is transmitted to be read in a field that doesn't allow for letters. Illinois may not allow the letter.
Based on the entries that you've made here the new account number is 11 digits plus that A. The letter that you've included from the account manager describes the new account number as an 11 digit number. So it would seem that the 11 digit number doesn't need the A in order to be the account number of record. I would think you could leave the A off. But if you are not certain then you should contact the account manager and make certain that Illinois requires that A.
@vaan
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