/s/ to /x/ sound change

Hello!

My question may seem a bit amateurish, but I am an amateur
in this field (being an software engineer by profession):
When reading the discussion on slavic endings I started to
wonder how easy/likely is the /s/ to /x/ sound change?
I know it happens, but these sounds are no so similar (well,
both voiceless fricatives), so this sound change is not that
trivial as palatalization, for example. I would rather imagine
/s/ > /S/ > /x/. What (phonetically) help such a change?

Jerzy

p.s. I follow this list for a quite a long time and I like it
very much, but never dared a post before.