Hi Olga, if you go into the configuration menu (three dots to the right of your Portfolio plan name) of your plan and select 'Issue Sources', you can run through the same steps again as when setting up your plan the first time: select sources, releases...(no need to change these in your case, just click next)
In the last step you should be able to (de-)select the issues you want to add to/remove from your scope.
Hope this helps
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