redirect all subdomains from http to https

As the Apache Wiki entry for RewriteHTTPToHTTPS states,

Using mod_rewrite to do this isn't the recommended behavior. See RedirectSSL

The vhost configuration for forced HTTP to HTTPS redirection - that also works with subdomains - is this:

<VirtualHost *:80>
    ServerName example.com
    ServerAlias *.example.com

    <Location "/">
        Redirect permanent "https://%{HTTP_HOST}%{REQUEST_URI}"
    </Location>
</VirtualHost>

<VirtualHost *:443>
    [...your vHost configuration...]
    SSLEngine On
    SSLCertificateFile /path/to/your/cert.pem
    SSLCertificateKeyFile /path/to/your/privkey.pem
</VirtualHost>

Explanation: Redirect and RedirectMatch normally don't have the variables (like {HTTP_HOST}) from Mod_Rewrite but if you use <Location > these Variables will be assigned.

Redirect permanent (Alternative: Redirect 301) will redirect with a http 301 code, because

The 301 redirect is considered a best practice for upgrading users from HTTP to HTTPS.

Note: This config is based on Wildcard Certificates for subdomains.

You could add this to your server's .conf file:

<VirtualHost *:80>
  ServerName subdomain.example.com
  ServerAlias *.example.com

  RewriteEngine On
  RewriteCond %{HTTP_HOST} ^(.+)\.example\.com$
  RewriteRule ^(.*)$ https://%1.example.com$1 [R=302,L]
</VirtualHost>

The ServerAlias will allow the vhost to act as a wildcard, then you can extract the subdomain(s) from the host header and include them in the rewrite to https

Here is a simpler universal modification to the .conf file:

<VirtualHost *:80>
    #...whatever you already have set up...

    RewriteEngine On
    RewriteCond %{HTTP_HOST} ^(.*)$ [NC]
    RewriteRule ^ https://%1%{REQUEST_URI} [L,NE,R=301]

</VirtualHost>