Can I undo a lead conversion after it's converted to an opportunity?

No, you can't revert or undo a lead conversion to its initial state after its converted to an opportunity. However, as a workaround, you can manually perform a few tasks to revert your action.

You can either delete or change the status of the opportunity to Lost and then create a new lead. Once the new lead is created, you can then either set the status of the initial lead to Retired or delete the lead from your sales application.