Strategy Question, 5.6 to 5.7 stepped migration

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I built a website in 5.6 for a non-profit. This is an arts/exhibition org that is 35 years old and has received funding to create a massive new archive section beginning over the next few months. See:http://cocaseattle.org

I planned to migrate their existing site to 5.7 sometime late this year, but without a migration script, it will be a huge task.

I don't wish to build the new archive in 5.6 as I want to move them forward, but I'm not ready and we don't have capacity to do a full migration now. If I build the archive in 5.7 in a subdomain, could I use that install for the eventual migration so that the archive and the main site run off of the same DB and files?

Veronikan