Creating a complete hidden site behind a public homepage

I’m designing a full website for a magazine. I’m confused about the best way of organising this because the website already has a live, but very basic, homepage with a call for submissions to the magazine on it. This does need to remain public.

When the first issue launches, that page will need to be replaced by a new homepage, with a menu bar linking to a full site (the page which is currently live doesn’t have a menu bar). I need to design this site in the background but can’t work out how to do it so all the pages are linked to a new, but temporarily hidden, homepage.

This might be very obvious and I’m sorry if that is the case! I have searched for similar questions but coudln’t find any. I want to be sure I start designing this in a logical way.

Many thanks in advance for any advice or help. I can privately give you a link to the site if necessary.

Lindsey