It should be possible using the command

sudo /etc/init.d/apache2 reload

As per best practice do the following when you add a new website:

1. Place the config file under /etc/apache2/sites-available in your example newsite.conf.
2. "Enable" the websitee, run the command: a2ensite newsite
3. Test your config, run the command apache2ctl configtest
4. at last reload the config: service apache2 reload

This won't cause an outage for other running websites. You did everything you can to avoid taking down the webserver. apache2ctl configtest will let you know if you did syntax error or a module you want to use in the config is not loaded yet, or the path you defined to an ssl certificate is not valid. Any open session from the clients will stay open. Although you do all necessary steps to avoid taking down the webserver, it can very well happen in case for instance an ssl certificate is not in a correct format, unfortunately apache2ctl configtest can't protect you from that.

I hope my answer helps!

Reload does a "Graceful Restart".

From the Apache documentation Stopping and Restarting :

The USR1 or graceful signal causes the parent process to advise the children to exit after their current request (or to exit immediately if they're not serving anything). The parent re-reads its configuration files and re-opens its log files. As each child dies off the parent replaces it with a child from the new generation of the configuration, which begins serving new requests immediately.

In theory no operations will be interrupted, but there will be a wave of server-processes restarting, with some (normally trivial enough) overhead.

In effect there are more variables involved, such as whether you are using prefork or worker, the number of children that you have specified, and in the case of worker the max threads per child.

Yes. conf.d files are included in Apache's configuration with a line like:

Include conf.d/*.conf

so changes are only noticed when the main configuration is reloaded. Note you can also use the 'graceful' restart (/etc/init.d/httpd graceful on some, apachectl graceful on others) to reload the configuration without dropping existing connections.

$ /etc/init.d/httpd reload

This will run a config syntax check and then will make apache reload its config files without interrupting traffic.

Should be possible using the command:

sudo /etc/init.d/apache2 reload

or

sudo service apache2 reload

More info on: http://www.electrictoolbox.com/article/apache/restart-apache/

On Windows, you can try:

httpd -k restart

Although I believe doing a graceful restart isn't supported on Windows. More info on: http://httpd.apache.org/docs/2.2/platform/windows.html#wincons

The main difference between the four different ways of stopping/restarting are what the main process does with its threads, and with itself.

Note that Apache recommends using apachectl -k as the command, and for systemd, the command is replaced by httpd -k

apachectl -k stop or httpd -k stop

This tells the process to kill all of its threads and then exit

apachectl -k graceful or httpd -k graceful

Apache will advise its threads to exit when idle, and then apache reloads the configuration (it doesn't exit itself), this means statistics are not reset.

apachectl -k restart or httpd -k restart

This is similar to stop, in that the process kills off its threads, but then the process reloads the configuration file, rather than killing itself.

apachectl -k graceful-stop or httpd -k graceful-stop

This acts like -k graceful but instead of reloading the configuration, it will stop responding to new requests and only live as long as old threads are around. Combining this with a new instance of httpd can be very powerful in having concurrent apaches running while updating configuration files.

Source: https://httpd.apache.org/docs/2.4/stopping.html

Recommendation: Use -k graceful unless there is something wrong with the main process itself, in which case a combination of -k stop and -k start or -k graceful-stop and -k start are the options of choice.