https://ibkr.info/node/873
Tax Reporting: How often may I change the tax basis method on my account?
You may change the tax basis methodology at any time by logging into Account Management and accessing the Tax Basis Declaration page in the Report Management menu. When you change the Tax Basis Declaration, the new methodology applies to trades made after the change; it does not affect existing trades.
By default, the tax basis methodology is set to First In, First Out (FIFO). Choose from First In, First Out (FIFO), Last In, First Out (LIFO), or Maximize Losses (ML). The tax basis declaration will appear in the Code column on your Activity Statement for trades where a methodology other than FIFO has been applied.