Virtual WP Update?

Hi All

I've been developing WP for a few years and have quite a few client and
personal sites built.
I know the necessity for updates, annoying, but essential to do.

My question is, does anyone know of a plugin/script/program etc..
that can do a virtual upgrade to the latest WP version without actually
doing it live?
In other words, I'd like something that will take a site, upgrade it so I can
have a look to see if it all functions, and then upgrade it for real after I see
that it still looks good.

I don't want to clone it or use xampp or other offline servers, just something
quick to view.
I've had a couple of WP sites mess up after an upgrade due to the theme I
was using. Not really concerned on personal sites, but hesitant on client sites.

I remember a couple years ago there was plugin I think that did upgrades for viewing
purposes but didn't save the upgrade until I wanted it for real. Does anybody
remember this? I couldn't find it anywhere in my searches.

Thanks
Jack