816 hybrid

My bad I mean 915 hybrid shaft

Hi Mark,
Yes they are indeed completely interchangeable. 

However,
816 H1 and H2 have a 1 degree SureFit Tour Hosel degree range.
915 H and Hd have a 0.75 degree SureFit Tour Hosel degree range. 
This means that if you take your 915 H shaft and put it into your 816 H head, your degree range each time you make an adjustment to a letter or number on the hosel will be 0.75 degrees.   Same as it was when it was in your 915 H or Hd head.

A good way to think about it is tell yourself that it is 'the hosel not the head' that keeps the SureFit degree range.

Hopefully this clears everything up for you.  If not, just let us know and we'll make sure that you are completely up-to-speed!

Regards,
Mitch D
Team Titleist Canada