In her most recent post Lorna Mitchell talks about her own experiences in getting a current application upgraded and ready to run on PHP7. It can best be summed up in a tweet from her: "Total lines of code change needed to make the @joindin API work on PHP7: zero"

With PHP7 looking increasingly stable (relatively speaking, it's still pre-alpha so it's VERY early days and anything could happen!), and work going well on the GoPHP7-ext project to get extensions converted, I have been thinking about the migration guides we'll need to help people upgrade their existing applications. To this end, I took the simplest project I currently have (http://api.joind.in) and gave it a whirl on PHP7, using Rasmus' PHP7 dev box. [...] All in all, it wasn't a great study of what kinds of things can go wrong when upgrading projects, because as far as I can tell with the test coverage that we have, it Just Works (TM).

She points out that a major contributing factor to it "just working" in PHP7 probably has to do with the few amount of dependencies. She also suggests looking at the tools you do use and see if they're already doing work to make it cooperate on PHP7 when the time comes. She describes some codebases that should "just work" with PHP7 including smaller codebases and things created with more modern tools/libraries/frameworks/etc.