A lose debian analogy, the difference being MS might revoke access to the Insider program without notice.
If you want Jessie (stable), you can upgrade free of charge from Wheezy (oldstable) or Squeeze (oldoldstable) but if you want to upgrade from Lenny or Etch or otherwise, you must buy a new computer with Jessie pre-installed or purchase retail Jessie DVD installation media.
On the other hand if you sell your soul to Linus, promising your firstborn child to RMS, you may access the Stretch (testing) and Sid (unstable) rolling releases free of charge but may not downgrade to Jessie without following the steps above.
If you are using Testing during the trunk freeze prior a new release, you may painlessly switch to Stable provided you had a valid installation of OldStable or OldOldStable.
windows translation of the above:
If you want Windows 10, you can upgrade free of charge from 8.x or 7 but if you want to upgrade from Vista or XP or otherwise, you must buy a new computer with Windows 10 pre-installed or purchase retail Windows 10 DVD installation media.
On the other hand if sign up to the Windows Insider program, promising your firstborn child to Microsoft, you may access the Slow or Fast rolling releases free of charge but may not downgrade to Windows 10 without following the steps above.
If you are using Windows Insider builds prior to the new release, you may painlessly switch to Windows 10 provided you had a valid installation of 8.x or 7.