is making changes effective October 1, 2008 which affect the size of Transaction ID values returned when payments are processed.

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What is going on with the Transaction ID field?
The Transaction ID field was originally developed with a maximum numeric value of 2,147,483,647. As the number of merchants using the Authorize.Net Payment Gateway has grown, we have identified a time in the near future in which the Transaction ID count will surpass 2,147,483,647. For this reason, we are in the process of expanding the range of Transaction IDs that the payment gateway can issue. Accordingly, we are communicating to all Authorize.Net merchants to verify that your systems can accommodate a 10-digit Transaction ID greater than 2,147,483,647.

From our testing on MySQL 4.1 and 5.0, Zen Cart v1.3.8a users will not witness any obvious ill effects as a result of change to longer Transaction IDs.

It is true that the data stored in the "authorizenet" table, which archives details of historical transactions, will not properly accommodate the longer lengths; however, it seems that the only side-effect is that the number gets stored literally as 2147483647. While that makes that particular value meaningless if it's supposed to be a higher value, at least it will not cause interruptions in customer shopping experiences. Additionally, since the Transaction ID value is stored elsewhere as a text-only comment in a text field, the Comments/Order-History area where the store administrator would actually see the Transaction ID for purposes of refunds etc will not be adversely affected.

For those who desire it, the quick solution to allow the full transaction_id to be stored in the authorizenet table is to simply run this SQL command on their database, either via the Install SQL Patch tool in their Zen Cart admin, or with phpMyAdmin (assuming you add any appropriate table-prefix if you choose to use phpMyAdmin):
  ALTER TABLE authorizenet CHANGE COLUMN transaction_id transaction_id varchar(20) NOT NULL default '';
Zen Cart versions newer than v1.3.8a will include this fix.

Additional discussion is found here: