With a little leg work you can convert a subfolder wp network into a subdomain wp network. I’ve done it myself so I know its possible. Easiest approach would be to
- backup files and database
- give yourself a cushion of time to deal with problems
- review the network setup instructions for sub-domain,
- properly point your subdomains to the wordpress folder
- make the subdomain changes in admin > Network Setup, wp-config, and htaccess
- use WPCLI to find+replace on database (old subfolder to new subdomain)
- troubleshoot any issues (hard coded urls, missing images, browser cached 301’s, serialized strings — depends on your setup)
Here’s an article on it:
Alternatively, (this is more of a round-about solution), but you can look at each sub-domain as simply another domain (because really thats all they are). Properly point your subdomains to your wordpress folder. You can then domain-map your subfolder sites to a sub-domain by changing each sites Site URL to the sub-domain or by using a domain mapping plugin.