Changing Main Site Mapping

I would like my main site URL to point to a sub domain.

Although I see the area to add mapping to the sub-domain, I am unsure about what will happen on the current main site. It doesn't have an "unmap" button.

Can I map the main site to the Sub domain without any problems?
What will happen to the current Main site mapping? Can I "create" a new sub domain for it?

Both site are currently in maintenance mode.