QB Issue Resolution:

Yes, you can change everything, even the expense account, except the amount and the bank account it comes from without affecting previous reconciliations

Resolution for Issue 'Can I edit the vendor and memo/description of a transaction after the transaction been reconciled?' available: Yes (Solved).
Source: Intuit Community forum.

Can I edit the vendor and memo/description of a transaction after the transaction been reconciled?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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