Change php Version upgrade wordpress installation

Hi!

I have a question about upgrading a 3.5.1 wordpress installation with 5.2. php version to wp 4.3.1. and changing the php version to 5.6.

When is the best time to change the php version?
Before the incremental upgrade or after? (and after a backup of course)
In other words:
Would wp 3.5.1 run on php 5.6 or does wp 4.3.1 run on 5.2?

Any help is appreciated!