Changing forum address (being a subdomain change or just moving to/away from the top level folder) is as simple as updating the url key in config.php.
Keep in mind that while this is true for core features, other extensions might have kept the old forum in memory. A php flarum cache:clear will most likely be needed. I've heard that depending on your settings Flagrow Upload might need some checks (if you're using it ?)
I suggest you make a copy of the database and Flarum files to the new address, so you can test what happens without disturbing your current install. Then when you're ready remove that copy, point the subdomain to your current installation and setup a permanent redirect including the path so every previous url goes to the new address.
Depending on what software you install on the root you might need to tune the redirects so they only redirect previous Flarum paths.
