Running 2 different WP versions/blogs on same domain

I’m having a bit of confusion about how to move an existing blog to a sub-directory of my domain and install a new blog as the primary domain without totally screwing things up. The second complicating factor is that each of these blogs are running different versions of WP — please don’t suggest upgrading, everything with break and it is what it is.

To move my existing blog to the /blog sub-directory, I was planning on following the instructions below:

https://smallbusiness.chron.com/move-wordpress-root-subfolder-52605.html

And to install my new blog to my primary domain, I was planning on following the instructions here:

https://www.ostraining.com/blog/wordpress/move-your-wordpress-site

However, I’m sure I’d overwrite my old site at the /blog location while installing my new primary domain blog. For various reasons, I’d rather use /blog as it will not incur additional plugin licensing fees while using blog.mydomain.com will.

What would be the best way to accomplish this? I’ve searched around and can’t find anything that would pertain to this particular situation with 2 different versions: one being primary and the other at the /blog location.

Thanks in advance for any help or suggestions!