The blurb for JIRA Portfolio describes how the tool can be used to sketch out multiple, alternative-future plans and then activate the chosen one (by synchronising with JIRA I guess). I take this to mean that you can have a master plan currently running, fork it off to draft a possible alternate plan, then declare that alternate plan to be the master plan once you're happy with it.
However, because the Plan owns the Team definitions, if any team settings change on the current master plan (e.g. new members, leave dates) while a new plan is in draft, you have to manually bring those changes across. When trying to plan for 200+ people, this is near impossible.
To plan large work programs in practice, I need to be able to migrate team, skills and stages data between plans. Much better though would be for that data to be factored-out of my plan, so that I can have two or more plans live-linking to a common set of Team records. I.e. give me the option of having plans that just use Teams, rather than own them.