Combining Free Content With Paid Content?

I currently run a couple Korean language websites. one is free, and the [url=]other is subscription-based. The reside on different domain names.

The free site was built first as a hobby years ago, and gets a significant amount of traffic from good rankings. A few years ago, I redid the entire thing including all lessons, included more content, more lessons, rewrote existing lessons, etc. and put them on the subscription-based site. Since all those lessons were put behind a login, SEO was difficult to do on the new site, and I simply redirect and try to sell the new site from the old site…However, the new site has significantly less traffic and eyes on it this way…traffic only arrives at it from links on the free site for those who bother to look into them - but I think it comes across more as “ads” to my own site rather than subscribing for more features and better content from the same company.

I’m trying to decide if it’s a good idea to combine the two. I’m considering moving all the free content from the old website and putting it on the subscription website somewhere (also redesigning the look of the newer site). This would bring traffic directly to that site (ultimately where I want people) and allow me to sell them better.

But does it make sense? The free content consists of lessons covering some of the same topics as the paid lessons, but the paid lessons contain more features such as audio and were completely re-written, and I think they are organized better and simply better lessons. Thus if users come to the combined site and view the free lessons, they may like them (as free) but may feel that a paid version wouldn’t be that much better, when in fact the paid ones are completely rewritten - just cover similar topics. It’s not like they are the exact same lessons with some features (such as audio) disabled. They are just lower quality lessons on similar topics when I first started years ago. But they do get traffic…

So I don’t know what is best to do marketing-wise. Ultimately, I want people to sign up for the paid lessons. It seems I have three options:

I can leave the two sites separate and simply continue to promote the paid one from the free one.

Or, I can move the free content to the newer site and do redirects to the new site (essentially a copy of all files / content to new site),

or I could come up with some ideas for completely new “free equivalents” of the paid lessons (disabling features, for example) and redirect from the old links to those completely new free equivalents (rather than simply the same lesson in a different location) trying to maintain that free traffic I’ve gained over the years somehow…

Any ideas?