Update: Ok, i give up. Nothing helps. Reinstalled K2 and reuploaded template override files. Didn't help. Created empty template with just top menu to reach K2 content - didn't help. I don't even have a clue where the problem is.
On other server i've tested K2 override with fresh Joomla installation and it worked just fine.

Ok, i finally solved my problem.
Thing is that if you use SH404 SEF component together with K2 component, then be sure you do not check "Yes" in "Use table-less output" in SH404 configuration, "SEO management" tab.
Because if you will choose this, then template override won't work.

Thank you for your replies!

I was having this same problem, only I'm *not* using SH404 on this site. But none of my changes to the template were showing up.

I went through everything on this thread and made tried to make sure I'd done everything suggested.

When I created a new override template it showed up fine in the K2 categories. But no changes were displaying on the frontend. Then I changed 'Inherit parameter options from category' to 'None' - and, sure enough, it fixed this.

So, yeah, I'm offering up my blockhead oversight to others. Inheriting parameters overrides the 'layout template'. An obvious mistake to be sure but if I save someone some frustration... [thumbs up].

Oh! Thanks man!
I thought I went crazy because it was always working before.

Problem is definitely sh404SEF's setting "Use table-less output" which must be set to "No". Then everything works as it must.