Yes you areright. If you are upgrading then the new priority names will not be available. But you can always go and edit the priority names.
Here's the link which confirms this
JIRA 6.4 ships with a new set of priorities, and a new resolution. The old default priorities of Blocker, Critical, Major, Minor and Trivial will be replaced with Highest, High, Medium, Low and Lowest. If you are upgrading an installation of JIRA, or importing existing data, JIRA will respect your original priorities and they will continue to be available instead of the new priorities. The new priorities will only become available and therefore the default in new installations.
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