I'm trying to enter my 529 acct # (schedule M) & TurboTax says the # should only contain numbers, but as of 27 Sep 24, they were assigned new #'s that start with an "A".

You could, or just leave the leading A off the Account Number entry on Schedule M.

 

If your Illinois State Return ultimately rejects because of this account number, then you definitely should leave the A out.  It doesn't appear that IL tax forms have been updated for the change, as I could not find 2024 Schedule M instructions.

 

Here's How to Contact your State Dept of Revenue for more info.

 

 

 

 

When Bright Start 529 transitioned to a new program manager on September 30, 2024, account holders were
assigned new account numbers. As such, each Bright Start 529 account that was open prior to September 27,
2024, has a minimum of two account numbers associated with it during the 2024 calendar year:
1. A 9-digit numeric account number valid through September 27, 2024
2. An 11-digit alphanumeric account number valid from September 30, 2024, onward

Will there be an update to the validation of that field ?

This Validation check is keeping the Illinois form being submitted electronically. Is my only option to print out the form and mail it in. Since the validation is not part of the form you should correct your issue.