I heard that Zach Braff is leaving scrubs, so if the show comes back for another season, will they keep the original cast?

I've only recently started to watch Scrubs, but I can't imagine how it would be without one of the main characters. I hope it isn't true.

Unfortunately it's true.
Zach is leaving Scrubs and the seventh season is his last one. Anyway he will be in some episodes of the eight season as a guest star and I think he's gonna direct some episodes.
If you search on the web you'll find quite a lot by now.

Can't think of Scrubs without Zach: he's the best! Hope the rest of the cast will remain the same.