Updating from a 1.8 backup directly to a 1.9 install?

Hello!

I have a semi broken 1.8 install (suddenly the majority of groups and content disappeared, not sure why but I suspect it might have something to do with me having updated a "Hostgator quickinstall" installation manually, although that was working fine for several months).

Anyways... so I have a backup of the site before the issues happend, i.e. a 1.8 .sql dump, a copy of the content directory and a copy of the public_html directory and I am thinking to directly update to Elgg 1.9 with it.

Preferably while having the original still somewhat working Elgg 1.8 install still online in a different subdirectory until the new site is fully working.

Would I need to do a new full 1.8 install first and then go through the regular update procedure, or is there some way to just point a new 1.9 install to an existing database and content directory without losing anything?

Thanks for the help and sorry if that question is a bit newbie like ;)