Is Multisite the only way to have a subdomain look like a completely different site with it's o

I am working on a fundraising campaign for a client whose site is currently built on WordPress. I would like to put together a site on a subdomain for them focusing on the fundraising efforts and the project status. I want this site to have a completely different look and feel. Is converting their current install to Multisite the only way to make this happen? I am scared of losing their current site trying to set this up and make it happen. I know that deactivating all the plugins is the first thing that has to happen before the network setup…that makes me VERY nervous. :expressionless: