We don't have an exact way to make a copy of a plan in the way you're describing. It sounds like you want to take a 'snapshot' of a plan, then a few months later compare your plan to the past?
As Portfolio for Jira always sync with the latest Jira data we don't track historic values.
The closest feature we have to do this is to use the scenarios feature. This helps doing "what if" scenario planning, and let's you have multiple versions of your plan at once. See https://confluence.atlassian.com/jiraportfolioserver/configuring-scenarios-868977946.html
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