Update theme while keeping old site up

Our school has a multisite domain so each teacher can have their own site. The school is school.com, the teachers are each jsmith.school.com, esmith.school.com, etc. To update theme for school.com there are going to be some major changes to locations, possible broken links, etc. So I want to keep old school.com up while doing the updates. Is there a way to make say new.school.com with all of the stuff from current school.com (the pages, posts, etc), test it, then when new one is ready just transfer it to school.com? Is this possibly an htaccess issue? Making a copy of school.com in a folder school.com/wp2, updating the wp2 version, then pointing school.com htaccess to it? Would that screw with jsmith.school.com?