Sara,
I just want to provide clarification to your issue in case you haven't contact your account manager. Hypothetically, if you currently have a subscription for 50 Desktop Advanced Concurrent Use licenses, you will also receive 50 Pro Named User licenses. You do not need to convert your Desktop licenses to Pro licenses and vice versa. For all Desktop users, nothing has change.
If you want to use ArcGIS Pro in addition to ArcGIS Desktop, you will need to go to AGOL, create an organization, create users, assign Pro licenses to users, install Pro and then sign into Pro using the AGOL account assign with Pro licenses. Again, this does not affect your Desktop licenses.
How long does it last? Because you have a perpetual license subscription, your Pro licenses are perpetual as well. If you choose to not renew your maintenance agreement, you will not have access to AGOL and therefore cannot use named user licensing option. However, you can go to MyEsri to convert your ArcGIS Pro Named User licenses to Concurrent Use licenses. It will create a perpetual concurrent user license for Pro. You can authorize the Pro licenses on your license manager, the same process for authorizing ArcGIS Desktop. Just to be clear, you are converting Pro named user licenses to Pro concurrent use licenses. You are not converting Desktop licenses.
Hope this clears up any confusion. If you have further questions, contact your account manager or Customer Service as Derek Law suggested above.