When you are performing drastic changes to your website, it is advisable to turn switch your Joomla website to Offline mode (Site > Global Configuration > Site Offline > Yes). Of course, the default offline message is ok - however, many times it's much better to create a specific message which is tailored for the needs of your specific Joomla website and audience.

The offline image can also be changed so that rather than having a Joomla logo - you can create a custom image of your own. We recommend putting up something amusing, so at least after the users get annoyed with the website being offline, at least they know you have a good sense of humour and hopefully come to check again later :)

Change the Offline message in the Site Settings of the Global Configuration, and insert your custom message. This is the basic configuration