[ESMA 70-872942901-38 MiFID II MiFIR market structures Q&A, Q&A 4.7]

Trading venues have discretion to set the rules covering the treatment of orders remaining on the book at the moment the minimum tick size increases, including whether or not such orders are to be cancelled or amended. Trading venues are responsible to disclose those rules appropriately. Trading venues must also observe the requirement to enforce the minimum tick size for orders submitted after that tick size comes into force.