Please read this FAQ for the key details concerning your Unity 5 Pro perpetual license, and your option to continue on the new version of the engine launching next year, Unity 2017.

Unity Pro now, next year and beyond

• Does Unity’s new subscription licensing mean that I have to pay for Unity 5 again?
• You do not have to pay again for the Unity 5 product cycle. You own your 5.x Pro perpetual license, and you can develop with it for as long as you like.
• Will I get all updates in Unity 5?
• Yes, as an owner of a Unity 5 Pro perpetual license, you will get all updates in the Unity 5 product cycle. We did state in the Unity 5 FAQ that your Unity 5 license expires in March 2017. However, don’t worry, should the release of 5.6 slip past March 2017 it will still be included in 5.x perpetual regardless. In addition, Unity 5.6 will receive patches for an additional 12 months (no new features though).
  
  A note about access to cloud services after the end of Unity 5. After we ship 5.6 and conclude the Unity 5 product cycle, you will no longer have access to Performance Reporting, or to Pro-tier cloud services unless you subscribe to Pro. Your Cloud Build, Analytics and Collaborate services will revert to the free (Personal) versions/tiers.
• How long is the Unity 5 product cycle?
• The 5 product cycle will be approximately two years.
• What’s coming up after Unity 5?
• After the end of the Unity 5 cycle, we’ll launch Unity 2017, a new product cycle that will reflect our model of continuous releases. Only subscribers will have access to the updates in Unity 2017.
  
  The offer that you received is an opportunity to get discounted access to updates in Unity 2017 for 12-24 months, by subscribing to the new Unity Pro. If you accept your offer, you start paying in April 2017 only.
• When does Unity 2017 launch?
• Unity 2017 will debut with a beta release available in April 2017.
• What features are coming in Unity 2017?
• For more details see the Unity 2017 blog post and the roadmap.