The investments in a shortlist cannot be edited once you have selected to finalize it. You can tell if a shortlist is finalized if it says "Recommendations finalized" below the name of the shortlist. This restriction is in place to ensure historical accuracy and so that all client accounts attached to investment change will reference the same research process and recommended investments.
An existing investment change can be canceled and a new investment change (and accompanying shortlist) created if you did this in error. You can cancel an investment change by opening the change and selecting a 'Change Status' of Cancelled.