Yes, you can convert the reconciled check to a payment, SRD5000.

Simply change the account to Accounts Payable and tag the name of the vendor who sent the check. Doing this will make the entry appear on the vendor transaction list as a credit. And, you can link it to a bill as a payment.

To apply the check:

1. Go to Vendors, then choose Pay Bills.
2. Mark the outstanding bill, then click Set Credits.
   
3. Mark the credit, then click Done.
   
4. Click Pay Selected Bills.

I've got this link for you: Bill shows as unpaid after writing a check in QuickBooks Desktop. It has 4 solutions that will help manage created checks to pay bills.

Upon sharing this, I still suggest consulting with your account. He/She might have specific instructions since the check has been reconciled already. Any changes to your account will affect the balance on your register.

Stay in touch if there's anything else you need. I'm always right here to help.