Can we change the reconciliation account in the vendor master data after transaction(s) have been posted to the vendor?
YES, the reconciliation account can still be changed. However, there are some prerequisites that need to be done first. I’d like to share the configuration changes and procedures:
A. Set the “Reconciliation Account” field in the vendor master as optional field.
Transaction code: OB23
Path: IMG → Financial Accounting (New) → Accounts Receivable and Accounts Payable → Vendor Accounts → Master Data → Define Screen Layout per Activity (Vendors).
Change both Accounting and Centrally part.
Set the status of “reconciliation account” field to optional entry from display only.
Save your work.
B. Define Adjustment Accounts for Changed Reconciliation Accounts
Transaction Code: OBBW
Path: IMG → Financial Accounting (New) → General Ledger Accounting (New) → Periodic Processing → Reclassify → Transfer and Sort Receivables and Payables → Define Adjustment Accounts for Changed Reconciliation Accounts.
Save your work.
C. Run Balance Sheet Adjustment Program
Transaction Code: F101
SAP SIMPLE Docs 
useful matter
our SAP implementing partner challenging no one can do this change once transaction been posted to vendor.