Article ID:17031 in the sage knowledgebase outlines this. IT sounds like all you need to do is reprint checks to void the original and assign the correct dates.

The information from this article is included below:

 Note that there are other articles referenced in teh event the check has been marked as cleared or reconciled in CM.

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To change the Check Date of a printed check in Accounts Payable, choose Option A. To change the Check Date of a manual check in Accounts Payable, choose Option B as follows:

Note: You cannot reprint or void a check that is cleared or reconciled in Cash Management. See article 22176 "How do I change the reconciled status of a transaction in Cash Management?" for more information on changing the reconciled status of a check.

Option A:

Use the Reprint Checks task (Tasks, Reprint Checks) to reprint your checks.  

Enter the correct Check Date in the Reprint Checks task. See article 17950 "How do I reprint Accounts Payable checks?" for more information on reprinting checks.

Note: Reprint Checks uses the original check date for the reversing entry.

Option B:

Void and delete the manual check using Specific date with the Void Payments task. (Tasks, Void Payments in versions 9.8 and earlier use Void Checks.)

Record the manual check again with the correct Check Date. See article 27601 "How do I correct an Accounts Payable manual check?" for more information on correcting a manual check.