Hi Ward ,
Thanks for your post.
We don't recommend working with old versions of our plugins. They may contain bugs and may be incompatible with current WooCommerce and WordPress versions.
We ensure our add-ons' current versions are compatible with the latest ATUM free version. They may work with older versions, but again, we don't recommend it and sometimes some new add-on features may require the newest ATUM free version. In your case, you must also update ATUM Free if you update Product Levels (indeed, the major part of the HPOS compatibility code is within the free plugin).
We didn't change anything in our subscription model. If you purchase Product Levels, you'll have access to the latest versions, the new updates and our premium support until your license expires. After that, you can continue using the plugins without any problems.
Regarding your question about using ATUM PL without ATUM Free, it isn't possible.
Best Regards,
José Andrés