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I have a Jira installation for 100 users, but only 25 of those users need the PDF exporting feature. Can I purchase a 25 user license for the app?
No. Your app license must exactly match your Jira license.

I have a valid app license for 25 users. How can I upgrade this to a 50 user license?
You will have to pay only the price difference between your previous and new license. Contact us, and we will send you all instructions. That's all.

How can I move my app license from one Jira server to another?
When migrating Jira to a new server in the recommended way with XML backups, your server ID and your app license key will not change. Thus you don't have to ask for a new app license, your existing app license will just work installed on the new server, too.

How can I upgrade my app license after upgrading from Jira 3 to Jira 4?
The app licenses generated for Jira 3 will not work on Jira 4. To get an updated license, ask us for a new license as written here and attach your old license to the request. If you have an active license for Jira 3, we will provide you with a Jira 4 compatible app license at no additional cost.
The only restriction is that updating your license to Jira 4 for free is available only at least 90 days after purchasing the license for Jira 3.