The FAQ was not clear about what happens to the Sonar software for customers who were subscribers since the beginning of its availability.
I was purchasing the yearly upgrades but moved to the subscription when it first became available.
If I remember correctly, once the subscription was available, Sonar was no longer being sold as a one-time purchase product.
I have Cakewalk projects dating back to 2004 and need to know what is going to happen to the Sonar software in the near future.
Will it become unusable or can I continue to update my old projects with the existing software in the next year then move forward?

Cakewalk has always offered their software with a one-time payment option. Recent versions of SONAR allowed the user to spread the payments across 12 months. Aside from this payment option, the only difference between the current version of SONAR and X3 and older versions is the activation method. At this time the activation methods have not changed. There is a plan should the activation servers get shut down to provide an alternate activation method.
 
There is no reason to expect SONAR to suddenly stop working in the near future. That said, there is no development going on so should something cause SONAR to stop working, it would be a problem.

In addition to scook's post, since there is no certainty about SONAR's usability - due to many factors other than Cakewalk's demise - it's a good idea to start exploring options for a new DAW, just in case.  I expect SONAR to be good for a long time to come, but better safe than sorry.

So, basically, I'm not paying for the software any more, not by my choice, but I'm able to keep it for free?

Once SONAR Home Studio/Artist/Professional or Platinum was purchased either using the one-time payment plan or by completed the 12 monthly payment plan, Cakewalk granted a non-expiring license to the software. There never was a requirement to continue making payments to use the software. Since the announcement, Cakewalk granted all users on the monthly payment plan a non-expiring license to the software.